The Parrot intermediate representation (PIR) is the primary way to program Parrot directly. It used to be an overlay on top of the far more primitive Parrot assembly language (PASM). However, PIR and PASM have since diverged semantically in a number of places and no longer are directly related to one another. PIR has many high-level features that will be familiar to programmers, such as basic operator syntax. However, it's still very low-level, and is closely tied to the underlying virtual machine. In fact, the Parrot developers specifically want to keep in that way for a number of reasons. PASM, the Parrot assembly language, is discussed in more detail in CHP-5Chapter 5.

As a convention, files containing pure PIR code generally have a .pir extension. PASM files typically end with .pasm. Compiled Parrot Bytecode (PBC) files have a .pbc extension. We'll talk more about PBC and PASM in later chapters.

PIR is well documented, both in traditional documentation and in instructional code examples. The project documentation in docs/ are good sources for information about the current syntax, semantics, and implementation. The test suite in t/compilers/imcc/ shows examples of proper working code. In fact, the test suite is the definitive PIR resource, because it shows how PIR actually works, even when the documentation may be out of date.

Statements

The syntax of statements in PIR is much more flexible then is commonly found in assembly languages, but is more rigid and "close to the machine" then some higher-level languages like C are. PIR has a very close relationship with the Parrot assembly language, PASM. PASM instructions, with some small changes and caveats, are valid PIR instructions. PIR does add some extra syntactic options to help improve readability and programmability, however. The statement delimiter for both PIR and PASM is a newline \n. Each statement has to be on its own line This isn't entirely true when you consider things like macros and heredocs, but we'll tackle those issues when we come to them., but empty whitespace lines between statements are okay. Statements may also start with a label, for use with jumps and branches. Comments are marked by a hash sign (#), and continue until the end of the line. POD blocks may be used for multi-line documentation. We'll talk about all these issues in more detail as we go.

To help with readability, PIR has some high-level constructs, including symbol operators:

  $I1 = 5                       # set $I1, 5

named variables:

  .param int count
  count = 5

and complex statements built from multiple keywords and symbol operators:

  if $I1 <= 5 goto LABEL        # le $I1, 5, LABEL

We will get into all of these in more detail as we go. Notice that PIR does not, and will not, have high-level looping structures like while or for loops. PIR has some support for basic if branching constructs, but will not support more complicated if/then/else branch structures. Because of these omissions PIR can become a little bit messy and unweildy for large programs. Luckily, there are a large group of high-level languages (HLL) that can be used to program Parrot instead. PIR is used primarily to write the compilers and libraries for these languages, while those languages can be used for writing larger and more complicated programs.

Directives

PIR has a number of directives, instructions which are handle specially by the parser to perform operations. Some directives specify actions that should be taken at compile-time. Some directives represent complex operations that require the generation of multiple PIR or PASM instructions. PIR also has a macro facility to create user-defined directives that are replaced at compile-time with the specified PIR code.

Directives all start with a . period. They take a variety of different forms, depending on what they are, what they do, and what arguments they take. We'll talk more about the various directives and about PIR macros in this and in later chapters.

Variables and Constants

Parrot Registers

PIR code has a variety of ways to store values while you work with them. Actually, the only place to store values is in a Parrot register, but there are multiple ways to work with these registers. Register names in PIR always start with a dollar sign, followed by a single character that shows whether it is an integer (I), numeric (N), string (S), or PMC (P) register, and then the number of the register:

  $S0 = "Hello, Polly.\n"
  print $S0

Integer (I) and Number (N) registers use platform-dependent sizes and limitations There are a few exceptions to this, we use platform-dependent behavior when the platforms behave sanely. Parrot will smooth out some of the bumps and inconsistencies so that behavior of PIR code will be the same on all platforms that Parrot supports. Both I and N registers are treated as signed quantities internally for the purposes of arithmetic. Parrot's floating point values and operations are all IEEE 754 compliant.

Strings (S) are buffers of data with a consistent formatting but a variable size. By far the most common use of S registers and variables is for storing text data. S registers may also be used in some circumstances as buffers for binary or other non-text data. However, this is an uncommon usage of them, and for most such data there will likely be a PMC type that is better suited to store and manipulate it. Parrot strings are designed to be very flexible and powerful, to account for all the complexity of human-readable (and computer-representable) text data.

The final data type is the PMC, a complex and flexible data type. PMCs are, in the world of Parrot, similar to what classes and objects are in object-oriented languages. PMCs are the basis for complex data structures and object-oriented behavior in Parrot. We'll discuss them in more detail in this and in later chapters.

Constants

As we've just seen, Parrot has four primary data types: integers, floating-point numbers, strings, and PMCs. Integers and floating-point numbers can be specified in the code with numeric constants in a variety of formats:

  $I0 = 42       # Integers are regular numeric constants
  $I1 = -1       # They can be negative or positive
  $I2 = 0xA5     # They can also be hexadecimal
  $I3 = 0b01010  # ...or binary

  $N0 = 3.14     # Numbers can have a decimal point
  $N1 = 4        # ...or they don't
  $N2 = -1.2e+4  # Numbers can also use scientific notation.

String literals are enclosed in single or double-quotes:

  $S0 = "This is a valid literal string"
  $S1 = 'This is also a valid literal string'

Strings in double-quotes accept all sorts of escape sequences using backslashes. Strings in single-quotes only allow escapes for nested quotes:

  $S0 = "This string is \n on two lines"
  $S0 = 'This is a \n one-line string with a slash in it'
  

Here's a quick listing of the escape sequences supported by double-quoted strings:

  \xhh        1..2 hex digits
  \ooo        1..3 oct digits
  \cX         Control char X
  \x{h..h}    1..8 hex digits
  \uhhhh      4 hex digits
  \Uhhhhhhhh  8 hex digits
  \a          An ASCII alarm character
  \b          An ASCII backspace character
  \t          A tab
  \n          A newline
  \v          A vertical tab
  \f          
  \r
  \e
  \\          A backslash
  \"          A quote

Or, if you need more flexibility, you can use a heredoc string literal:

  $S2 = << "End_Token"

  This is a multi-line string literal. Notice that
  it doesn't use quotation marks. The string continues
  until the ending token (the thing in quotes next to
  the << above) is found. The terminator must appear on
  its own line, must appear at the beginning of the
  line, and may not have any trailing whitespace.

  End_Token

Strings: Encodings and Charsets

Strings are complicated. We showed three different ways to specify string literals in PIR code, but that wasn't the entire story. It used to be that all a computer system needed was to support the ASCII charset, a mapping of 128 bit patterns to symbols and English-language characters. This was sufficient so long as all computer users could read and write English, and were satisfied with a small handful of punctuation symbols that were commonly used in English-speaking countries. However, this is a woefully insufficient system to use when we are talking about software portability between countries and continents and languages. Now we need to worry about several character encodings and charsets in order to make sense out of all the string data in the world.

Parrot has a very flexible system for handling and manipulating strings. Every string is associated with an encoding and a character set (charset). The default for Parrot is 8-bit ASCII, which is simple to use and is almost universally supported. However, support is built in to have other formats as well.

Double-quoted string constants, like the ones we've seen above, can have an optional prefix specifying the charset or both the encoding and charset of the string. Parrot will maintain these values internally, and will automatically convert strings when necessary to preserve the information. String prefixes are specified as encoding:charset: at the front of the string. Here are some examples:

  $S0 = utf8:unicode:"Hello UTF8 Unicode World!"
  $S1 = utf16:unicode:"Hello UTF16 Unicode World!"
  $S2 = ascii:"This is 8-bit ASCII"
  $S3 = binary:"This is treated as raw unformatted binary"

The binary: charset treats the string as a buffer of raw unformatted binary data. It isn't really a "string" per se because binary data isn't treated as if it contains any readable characters. These kinds of strings are useful for library routines that return large amounts of binary data that doesn't easily fit into any other primitive data type.

Notice that only double-quoted strings can have encoding and charset prefixes like this. Single-quoted strings do not support them.

When two types of strings are combined together in some way, such as through concatenation, they must both use the same character set an encoding. Parrot will automatically upgrade one or both of the strings to use the next highest compatible format, if they aren't equal. ASCII strings will automatically upgrade to UTF-8 strings if needed, and UTF-8 will upgrade to UTF-16. Handling and maintaining these data and conversions all happens automatically inside Parrot, and you the programmer don't need to worry about the details.

Named Variables

Calling a value "$S0" isn't very descriptive, and usually it's a lot nicer to be able to refer to values using a helpful name. For this reason Parrot allows registers to be given temporary variable names to use instead. These named variables can be used anywhere a register would be used normally ...because they actually are registers, but with fancier names. They're declared with the .local statement which requires a variable type and a name:

  .local string hello
  set hello, "Hello, Polly.\n"
  print hello

This snippet defines a string variable named hello, assigns it the value "Hello, Polly.\n", and then prints the value. Under the hood these named variables are just normal registers of course, so any operation that a register can be used for a named variable can be used for as well.

The valid types are int, num, string, and pmc Also, you can use any predefined PMC class name like BigNum or LexPad. We'll talk about classes and PMC object types in a little bit.. It should come as no surprise that these are the same as Parrot's four built-in register types. Named variables are valid from the point of their definition to the end of the current subroutine.

The name of a variable must be a valid PIR identifier. It can contain letters, digits and underscores but the first character has to be a letter or an underscore. There is no limit to the length of an identifier, especially since the automatic code generators in use with the various high-level languages on parrot tend to generate very long identifier names in some situations. Of course, huge identifier names could cause all sorts of memory allocation problems or inefficiencies during lexical analysis and parsing. You should push the limits at your own risk.

Register Allocator

Now's a decent time to talk about Parrot's register allocator it's also sometimes humorously referred to as the "register alligator", due to an oft-repeated typo and the fact that the algorithm will bite you if you get too close to it. When you use a register like $P5, you aren't necessarily talking about the fifth register in memory. This is important since you can use a register named $P10000000 without forcing Parrot to allocate an array of ten million registers. Instead Parrot's compiler front-end uses an allocation algorithm which turns each individual register referenced in the PIR source code into a reference to an actual memory storage location. Here is a short example of how registers might be mapped:

  $I20 = 5       # First register, I0
  $I10000 = 6    # Second register, I1
  $I13 = 7       # Third register, I2

The allocator can also serve as a type of optimization. It performs a lifetime analysis on the registers to determine when they are being used and when they are not. When a register stops being used for one thing, it can be reused later for a different purpose. Register reuse helps to keep Parrot's memory requirements lower, because fewer unique registers need to be allocated. However, the downside of the register allocator is that it takes more time to execute during the compilation phase. Here's an example of where a register could be reused:

  .sub main
    $S0 = "hello "
    print $S0
    $S1 = "world!"
    print $S1
  .end

We'll talk about subroutines in more detail in the next chapter. For now, we can dissect this little bit of code to see what is happening. The .sub and .end directives demarcate the beginning and end of a subroutine called main. This convention should be familiar to C and C++ programmers, although it's not required that the first subroutine or any subroutine for that matter be named "main". In this code sequence, we assign the string "hello " to the register $S0 and use the print opcode to display it to the terminal. Then, we assign a second string "world!" to a second register $S1, and then print that to the terminal as well. The resulting output of this small program is, of course, the well-worn salutation hello world!.

Parrot's compiler and register allocator are smart enough to realize that the two registers in the example above, $S0 and $S1 are used exclusively of one another. $S0 is assigned a value in line 2, and is read in line 3, but is never accessed after that. So, Parrot determines that its lifespan ends at line 3. The register $S1 is used first on line 4, and is accessed again on line 5. Sinec these two do not overlap, Parrot's compiler can determine that it can use only one register for both operations. This saves the second allocation. Notice that this code with only one register performs identically to the previous example:

  .sub main
    $S0 = "hello "
    print $S0
    $S0 = "world!"
    print $S0
  .end

In some situations it can be helpful to turn the allocator off and avoid expensive optimizations. Such situations are subroutines where there are a small fixed number of registers used, when variables are used throughout the subroutine and should never be reused, or when some kind of pointer reference needs to be made to the register this happens in some NCI calls that take pointers and return values. To turn off the register allocator for certain variables, you can use the :unique_reg modifier:

  .local pmc MyUniquePMC :unique_reg

Notice that :unique_reg shouldn't affect the behavior of Parrot, but instead only changes the way registers are allocated. It's a trade off between using more memory in exchange for less time spent optimizing the subroutine.

PMC variables

PMC registers and variables act much like any integer, floating-point number, or string register or variable, but you have to instantiate a new PMC object into a type before you use it. The new instruction creates a new PMC of the specified type:

  $P0 = new 'PerlString'     # This is how the Perl people do it
  $P0 = "Hello, Polly.\n"
  print $P0

This example creates a PerlString object, stores it in the PMC register $P0, assigns the value "Hello, Polly.\n" to it, and prints it. With named variables the type passed to the .local directive is either the generic pmc or a type compatible with the type passed to new:

  .local PerlString hello    # or .local pmc hello
  hello = new 'PerlString'
  hello = "Hello, Polly.\n"
  print hello

PIR is a dynamic language, and that dynamicism is readily displayed in the way PMC values are handled. Primitive registers like strings, numbers, and integers perform a special action called autoboxing when they are assigned to a PMC. Autoboxing is when a primative scalar type is automatically converted to a PMC object type. There are PMC classes for String, Number, and Integer which can be quickly converted to and from primitive int, number, and string types. Notice that the primative types are in lower-case, while the PMC classes are capitalized. If you want to box a value explicitly, you can use the box opcode:

  $P0 = new 'Integer'       # The boxed form of int
  $P0 = box 42
  $P1 = new 'Number'        # The boxed form of num
  $P1 = box 3.14
  $P2 = new 'String'        # The boxed form of string
  $P2 = "This is a string!" 

The PMC classes Integer, Number, and String are thin overlays on the primative types they represent. However, these PMC types have the benefit of the VTABLE interface. VTABLEs are a standard API that all PMCs conform to for performing standard operations. These PMC types also have special custom methods available for performing various operations, they may be passed as PMCs to subroutines that only expect PMC arguments, and they can be subclassed by a user-defined type. We'll discuss all these complicated topics later in this chapter and in the next chapter. We will discuss PMC and all the details of their implementation and interactions in CHP-11 Chapter 11.

Named Constants

The .const directive declares a named constant. It's very similar to .local, and requires a type and a name. The value of a constant must be assigned in the declaration statement. As with named variables, named constants are visible only within the compilation unit where they're declared. This example declares a named string constant hello and prints the value:

  .const string hello = "Hello, Polly.\n"
  print hello

Named constants may be used in all the same places as literal constants, but have to be declared beforehand:

  .const int the_answer = 42        # integer constant
  .const string mouse = "Mouse"     # string constant
  .const num pi = 3.14159           # floating point constant

In addition to normal local constants, you can also specify a global constant which is accessible from everywhere in the current code file:

  .globalconst int days = 365

Currently there is no way to specify a PMC constant in PIR source code, although a way to do so may be added in later versions of Parrot.

Symbol Operators

PIR has many other symbol operators: arithmetic, concatenation, comparison, bitwise, and logical. All PIR operators are translated into one or more Parrot opcodes internally, but the details of this translation stay safely hidden from the programmer. Consider this example snippet:

  .local int sum
  sum = $I42 + 5
  print sum
  print "\n"

The statement sum = $I42 + 5 translates to the equivalent statement add sum, $I42, 5. This in turn will be translated to an equivalent PASM instruction which will be similar to add I0, I1, 5. Notice that in the PASM instruction the register names do not have the $ symbol in front of them, and they've already been optimized into smaller numbers by the register allocator. The exact translation from PIR statement to PASM instruction isn't too important Unless you're hacking on the Parrot compiler!, so we don't have to worry about it for now. We will talk more about PASM, its syntax and its instruction set in Chapter 5. Here are examples of some PIR symbolic operations:

  $I0 = $I1 + 5      # Addition
  $N0 = $N1 - 7      # Subtraction
  $I3 = 4 * 6        # Multiplication
  $N7 = 3.14 / $N2   # Division
  $S0 = $S1 . $S2    # String concatenation

PIR also provides automatic assignment operators such as +=, -=, and >>=. These operators help programmers to perform common manipulations on a data value in place, and save a few keystrokes while doing them.

A complete list of PIR operators is available in CHP-13 Chapter 13.

= and Type Conversion

We've mostly glossed over the behavior of the = operator, although it's a very powerful and important operator in PIR. In it's most simple form, = stores a value into one of the Parrot registers. We've seen cases where it can be used to assign a string value to a string register, or an integer value to an int register, or a floating point value into a number register, etc. However, the = operator can be used to assign any type of value into any type of register, and Parrot will handle the conversion for you automatically:

  $I0 = 5     # Integer. 5
  $S0 = $I0   # Stringify. "5"
  $N0 = $S0   # Numify. 5.0
  $I0 = $N0   # Intify. 5

Notice that conversions between the numeric formats and strings only makes sense when the value to convert is a number.

  $S0 = "parrot"
  $I0 = $S0        # 0

We've also seen an example earlier where a string literal was set into a PMC register that had a type String. This works for all the primitive types and their autoboxed PMC equivalents:

  $P0 = new 'Integer'
  $P0 = 5
  $S0 = $P0      # Stringify. "5"
  $N0 = $P0      # Numify. 5.0
  $I0 = $P0      # De-box. $I0 = 5

  $P1 = new 'String'
  $P1 = "5 birds"
  $S1 = $P1      # De-box. $S1 = "5 birds"
  $I1 = $P1      # Intify. 5
  $N1 = $P1      # Numify. 5.0

  $P2 = new 'Number'
  $P2 = 3.14
  $S2 = $P2      # Stringify. "3.14"
  $I2 = $P2      # Intify. 3
  $N2 = $P2      # De-box. $N2 = 3.14

Labels

Any line in PIR can start with a label definition like LABEL:, but label definitions can also stand alone on their own line. Labels are like flags or markers that the program can jump to or return to at different times. Labels and jump operations (which we will discuss a little bit later) are one of the primary methods to change control flow in PIR, so it is well worth understanding.

Labels are most often used in branching instructions, which are used to implement high level control structures by our high-level language compilers.

Compilation Units

Compilation units in PIR are roughly equivalent to the subroutines or methods of a high-level language. Though they will be explained in more detail later, we introduce them here because all code in a PIR source file must be defined in a compilation unit. We've already seen an example for the simplest syntax for a PIR compilation unit. It starts with the .sub directive and ends with the .end directive:

  .sub main
      print "Hello, Polly.\n"
  .end

Again, we don't need to name the subroutine main, it's just a common convention. This example defines a compilation unit named main that prints a string "Hello, Polly.". The first compilation unit in a file is normally executed first but you can flag any compilation unit as the first one to execute with the :main marker.

  .sub first
      print "Polly want a cracker?\n"
  .end

  .sub second :main
      print "Hello, Polly.\n"
  .end

This code prints out "Hello, Polly." but not "Polly want a cracker?". This is because the function second has the :main flag, so it is executed first. The function first, which doesn't have this flag is never executed. However, if we change around this example a little:

  .sub first :main
      print "Polly want a cracker?\n"
  .end

  .sub second
      print "Hello, Polly.\n"
  .end

The output now is "Polly want a cracker?". Execution in PIR starts at the :main function and continues until the end of that function only. If you want to do more stuff if your program, you will need to call other functions explicitly.

CHP-04Chapter 4 goes into much more detail about compilation units and their uses.

Flow Control

Flow control in PIR is done entirely with conditional and unconditional branches to labels. This may seem simplistic and primitive, but remember that PIR is a thin overlay on the assembly language of a virtual processor, and is intended to be a simple target for the compilers of various. high-level languages. High level control structures are invariably linked to the language in which they are used, so any attempt by Parrot to provide these structures would work well for some languages but would require all sorts of messy translation in others. The only way to make sure all languages and their control structures can be equally accommodated is to simply give them the most simple and fundamental building blocks to work with. Language agnosticism is an important design goal in Parrot, and creates a very flexible and powerful development environment for our language developers.

The most basic branching instruction is the unconditional branch: goto.

  .sub _main
      goto L1
      print "never printed"
  L1:
      print "after branch\n"
      end
  .end

The first print statement never runs because the goto always skips over it to the label L1.

The conditional branches combine if or unless with goto.

  .sub _main
      $I0 = 42
      if $I0 goto L1
      print "never printed"
  L1: print "after branch\n"
      end
  .end

In this example, the goto branches to the label L1 only if the value stored in $I0 is true. The unless statement is quite similar, but branches when the tested value is false. An undefined value, 0, or an empty string are all false values. Any other values are considered to be true values.

The comparison operators (<, <=, ==, !=, >, >=) can combine with if ... goto. These branch when the comparison is true:

  .sub _main
      $I0 = 42
      $I1 = 43
      if $I0 < $I1 goto L1
      print "never printed"
  L1:
      print "after branch\n"
      end
  .end

This example compares $I0 to $I1 and branches to the label L1 if $I0 is less than $I1. The if $I0 < $I1 goto L1 statement translates directly to the PASM lt branch operation.

The rest of the comparison operators are summarized in CHP-13-SECT-3"PIR Instructions" in Chapter 11.

PIR has no special loop constructs. A combination of conditional and unconditional branches handle iteration:

  .sub _main
      $I0 = 1               # product
      $I1 = 5               # counter

  REDO:                     # start of loop
      $I0 = $I0 * $I1
      dec $I1
      if $I1 > 0 goto REDO  # end of loop

      print $I0
      print "\n"
      end
  .end

This example calculates the factorial 5!. Each time through the loop it multiplies $I0 by the current value of the counter $I1, decrements the counter, and then branches to the start of the loop. The loop ends when $I1 counts down to 0 so that the if doesn't branch to REDO. This is a do while-style loop with the condition test at the end, so the code always runs the first time through.

For a while-style loop with the condition test at the start, use a conditional branch together with an unconditional branch:

  .sub _main
      $I0 = 1               # product
      $I1 = 5               # counter

  REDO:                     # start of loop
      if $I1 <= 0 goto LAST
      $I0 = $I0 * $I1
      dec $I1
      goto REDO
  LAST:                     # end of loop

      print $I0
      print "\n"
      end
  .end

This example tests the counter $I1 at the start of the loop. At the end of the loop, it unconditionally branches back to the start of the loop and tests the condition again. The loop ends when the counter $I1 reaches 0 and the if branches to the LAST label. If the counter isn't a positive number before the loop, the loop never executes.

Any high-level flow control construct can be built from conditional and unconditional branches, because this is the way almost all computer hardware operates at the lowest-level, so all modern programming languages use branching constructs to implement their most complex flow control devices.

Fortunately, libraries of macros have been developed that can implement more familiar syntax for many of these control structures. We will discuss these libraries in more detail in CHP-6 "PIR Standard Library".

Classes and Objects

It may seem more appropriate for a discussion of PIR's support for classes and objects to reside in its own chapter, instead of appearing in a generic chapter about PIR programming "basics". However, part of PIR's core functionality is its support for object-oriented programming. PIR doesn't use all the fancy syntax as other OO languages, and it doesn't even support all the features that most modern OO languages have. What PIR does have is support for some of the basic structures and abilities, the necessary subset to construct richer and higher-level object systems.

PMCs as Classes

PMCs aren't exactly "classes" in the way that this term is normally used. They are polymorphic data items that can be one of a large variety of predefined types. As we have seen briefly, and as we will see in more depth later, PMCs have a standard interface called the VTABLE interface. VTABLEs are a standard list of functions that all PMCs implement or, PMCs can choose not to implement each interface explicitly and instead let Parrot call the default implementations.

VTABLEs are very strict: There are a fixed number with fixed names and fixed argument lists. You can't just create any random VTABLE interface that you want to create, you can only make use of the ones that Parrot supplies and expects. To circumvent this limitation, PMCs may have METHODS in addition to VTABLEs. METHODs are arbitrary code functions that can be written in C, may have any name, and may implement any behavior.

VTABLE Interfaces

Internally, all operations on PMCs are performed by calling various VTABLE interfaces.

Class and Object PMCs

The details about various PMC classes are managed by the Class PMC. Class PMCs contain information about the class, available methods, the inheritance hierarchy of the class, and various other details. Classes can be created with the newclass opcode:

  $P0 = newclass "MyClass"

Once we have created the class PMC, we can instantiate objects of that class using the new opcode. The new opcode takes either the class name or the Class PMC as an argument:

  $P1 = new $P0        # $P0 is the Class PMC
  $P2 = new "MyClass"  # Same

The new opcode can create two different types of PMC. The first type are the built-in core PMC classes. The built-in PMCs are written in C and cannot be extended from PIR without subclassing. However, you can also create user-defined PMC types in PIR. User-defined PMCs use the Object PMC type for instantiation. Object PMCs are used for all user-defined type and keep track of the methods and VTABLE override definitions. We're going to talk about methods and VTABLE overrides in the next chapter.

Subclassing PMCs

Existing built-in PMC types can be subclassed to associate additional data and methods with that PMC type. Subclassed PMC types act like their PMC base types, by sharing the same VTABLE methods and underlying data types. However, the subclass can define additional methods and attribute data storage. If necessary new VTABLE interfaces can be defined in PIR and old VTABLE methods can be overriden using PIR. We'll talk about defining methods and VTABLE interface overrides in the next chapter.

Creating a new subclass of an existing PMC class is done using the subclass keyword:

  # create an anonymous subclass
  $P0 = subclass 'ResizablePMCArray'

  # create a subclass named "MyArray"
  $P0 = subclass 'ResizablePMCArray', 'MyArray'

This returns a Class PMC which can be used to create and modify the class by adding attributes or creating objects of that class. You can also use the new class PMC to create additional subclasses:

  $P0 = subclass 'ResizablePMCArray', 'MyArray'
  $P1 = subclass $P0, 'MyOtherArray'

Once you have created these classes, you can create them like normal with the new keyword:

  $P0 = new 'MyArray'
  $P1 = new 'MyOtherArray'

Attributes

Classes and subclasses can be given attributes in addition to methods, which we will talk about in the next chapter which are named data fields. Attributes are created with the addattribute opcode, and can be set and retrieved with the setattribute and getattribute opcodes respectively:

  # Create the new class with two attributes
  $P0 = newclass 'MyClass'
  addattribute $P0, 'First'
  addattribute $P0, 'Second'

  # Create a new item of type MyClass
  $P1 = new 'MyClass'

  # Set values to the attributes
  setattribute $P1, 'First', 'First Value'
  setattribute $P1, 'Second', 'Second Value'

  # Get the attribute values
  $S0 = getattribute $P1, 'First'
  $S1 = getattribute $P1, 'Second'

Those values added as attributes don't need to be strings, even though both of the ones in the example are. They can be integers, numbers or PMCs too.

Input and Output

Like almost everything else in Parrot, input and output are handled by PMCs. Using the print opcode or the say opcode like we've already seen in some examples does this internally without your knowledge. However, we can do it explicitly too. First we'll talk about basic I/O, and then we will talk about using PMC-based filehandles for more advanced operations.

Basic I/O Opcodes

We've seen print and say. These are carry-over artifacts from Perl, when Parrot was simply the VM backend to the Perl 6 language. print prints the given string argument, or the stringified form of the argument, if it's not a string, to standard output. say does the same thing but also appends a trailing newline to it. Another opcode worth mentioning is the printerr opcode, which prints an argument to the standard error output instead.

We can read values from the standard input using the read and readline ops. read takes an integer value and returns a string with that many characters. readline reads an entire line of input from the standard input, and returns the string without the trailing newline. Here is a simple echo program that reads in characters from the user and echos them to standard output:

  .sub main
    loop_top:
      $S0 = read 10
      print $S0
      goto loop_top
  .end

Filehandles

The ops we have seen so far are useful if all your I/O operations are limited to the standard streams. However, there are plenty of other places where you might want to get data from and send data to. Things like files, sockets, and databases all might need to have data sent to them. These things can be done by using a file handle.

Filehandles are PMCs that describe a file and keep track of an I/O operations internal state. We can get Filehandles for the standard streams using dedicated opcodes:

  $P0 = getstdin    # Standard input handle
  $P1 = getstdout   # Standard output handle
  $P2 = getstderr   # Standard error handle

If we have a file, we can create a handle to it using the open op:

  $P0 = open "my/file/name.txt"

We can also specify the exact mode that the file handle will be in:

  $P0 = open "my/file/name.txt", "wa"

The mode string at the end should be familiar to C programmers, because they are mostly the same values:

  r  : read
  w  : write
  wa : append
  p  : pipe

So if we want a handle that we can read and write to, we write the mode string "rw". If we want to be able to read and write to it, but we don't want write operations to overwrite the existing contents, we use "rwa" instead.

When we are done with a filehandle that we've created, we can shut it down with the close op. Notice that we don't want to be closing any of the standard streams.

  close $P0

With a filehandle, we can perform all the same operations as we could earlier, but we pass the filehandle as an additional argument to tell the op where to write or read the data from.

  print "hello"       # Write "hello!" to STDOUT

  $P0 = getstdout
  print $P0, "hello"  # Same, but more explicit

  say $P0, " world!"  # say to STDOUT

  $P1 = open "myfile.txt", "wa"
  print $P1, "foo"    # Write "foo" to myfile.txt

Filehandle PMCs

Let's see a little example of a program that reads in data from a file, and prints it to STDOUT.

  .sub main
    $P0 = getstdout
    $P1 = open "myfile.txt", "r"
    loop_top:
      $S0 = readline $P1
      say $P0, $S0
      if $P1 goto loop_top
    close $P1
  .end

This example shows that treating a filehandle PMC like a boolean value returns whether or not we have reached the end of the file. A true return value means there is more file to read. A false return value means we are at the end. In addition to this behavior, Filehandle PMCs have a number of methods that can be used to perform various operations.

$P0.'open'(STRING filename, STRING mode)

Opens the filehandle. Takes two optional strings: the name of the file to open and the open mode. If no filename is given, the previous filename associated with the filehandle is opened. If no mode is given, the previously-used mode is used.

  $P0 = new 'Filehandle'
  $P0.'open'("myfile.txt", "r")
  
  $P0 = open "myfile.txt", "r"   # Same!
  

The open opcode internally creates a new filehandle PMC and calls the 'open'() method on it. So even though the above two code snippets act in an identical way, the later one is a little more concise to write. The caveat is that the open opcode creates a new PMC for every call, while the 'open'() method call can reuse an existing filehandle PMC for a new file.

$P0.'isatty'()

Returns a boolean value whether the filehandle is a TTY terminal

$P0.'close'()

Closes the filehandle. Can be reopened with .'open' later.

  $P0.'close'()
  
  close $P0   # Same
  

The close opcode calls the 'close'() method on the Filehandle PMC internally, so these two calls are equivalent.

$P0.'is_closed'()

Returns true if the filehandle is closed, false if it is opened.

$P0.'read'(INTVAL length)

Reads length bytes from the filehandle.

  $S0 = read $P0, 10
  
  $P0.'read'(10)

The two calls are equivalent, and the read opcode calls the 'read'() method internally.

$P0.'readline'()

Reads an entire line (up to a newline character or EOF) from the filehandle.

$P0.'readline_interactive'(STRING prompt)

Displays the string prompt and then reads a line of input.

$P0.'readall'(STRING name)

Reads the entire file name into a string. If the filehandle is closed, it will open the file given by name, read the entire file, and then close the handle. If the filehandle is already open, name should not be passed (it is an optional parameter).

$P0.'flush'()

Flushes the buffer

$P0.'print'(PMC to_print)

Prints the given value to the filehandle. The print opcode uses the 'print'() method internally.

  print "Hello"
  
  $P0 = getstdout
  print $P0, "Hello!"    # Same
  
  $P0.'print'("Hello!")  # Same

$P0.'puts'(STRING to_print)

Prints the given string value to the filehandle

$P0.'buffer_type'(STRING new_type)

If new_type is given, changes the buffer to the new type. If it is not, returns the current type. Acceptable types are:

  unbuffered
  line-buffered
  full-buffered

$P0.'buffer_size'(INTVAL size)

If size is given, set the size of the buffer. If not, returns the size of the current buffer.

$P0.'mode'()

Returns the current file access mode.

$P0.'encoding'(STRING encoding)

Sets the filehandle's string encoding to encoding if given, returns the current encoding otherwise.

$P0.'eof'()

Returns true if the filehandle is at the end of the current file, false otherwise.

$P0.'get_fd'()

Returns the integer file descriptor of the current file, but only on operating systems that use file descriptors. Returns -1 on systems that do not support this.

Exceptions

Parrot includes a robust exception mechanism that is not only used internally to implement a variety of control flow constructs, but is also available for use directly from PIR code. Exceptions, in as few words as possible, are error conditions in the program. Exceptions are thrown when an error occurs, and they can be caught by special routines called handlers. This enables Parrot to recover from errors in a controlled way, instead of crashing and terminating the process entirely.

Exceptions, like most other data objects in Parrot, are PMCs. They contain and provide access to a number of different bits of data about the error, such as the location where the error was thrown (including complete backtraces), any annotation information from the file, and other data.

Throwing Exceptions

Many exceptions are used internally in Parrot to indicate error conditions. Opcodes such as die and warn throw exceptions internally to do what they are supposed to do. Other opcodes such as div throw exceptions only when an error occurs, such as an attempted division by zero.

Exceptions can also be thrown manually using the throw opcode. Here's an example:

  $P0 = new 'Exception'
  throw $P0

This throws the exception object as an error. If there are any available handlers in scope, the interpreter will pass the exception object to the handler and continue execution there. If there are no handlers available, Parrot will exit.

Exception Attributes

Since Exceptions are PMC objects, they can contain a number of useful data items. One such data item is the message:

  $P0 = new 'Exception'
  $P1 = new 'String'
  $P1 = "this is an error message for the exception"
  $P0["message"] = $P1

Another is the severity and the type:

  $P0["severity"] = 1   # An integer value
  $P0["type"] = 2       # Also an Integer

Finally, there is a spot for additional data to be included:

  $P0["payload"] = $P2  # Any arbitrary PMC

Exception Handlers

Exception handlers are labels in PIR code that can be jumped to when an exception is thrown. To list a label as an exception handler, the push_eh opcode is used. All handlers exist on a stack. Pushing a new handler adds it to the top of the stack, and using the pop_eh opcode pops the handler off the top of the stack.

  push_eh my_handler
    # something that might cause an error

  my_handler:
    # handle the error here

Catching Exceptions

The exception PMC that was thrown can be caught using the .get_results() directive. This returns the Exception PMC object that was thrown from inside the handler:

  my_handler:
    .local pmc err
    .get_results(err)

With the exception PMC available, the various attributes of that PMC can be accessed and analyzed for additional information about the error.

Exception Handler PMCs

Like all other interesting data types in Parrot, exception handlers are a PMC type. When using the syntax above with push_eh LABEL, the handler PMC is created internally by Parrot. However, you can create it explicitly too if you want:

  $P0 = new 'ExceptionHandler'
  set_addr $P0, my_handler
  push_eh $P0
  ...

  my_handler:
    ...

Rethrowing and Exception Propagation

Exception handlers are nested and are stored in a stack. This is because not all handlers are intended to handle all exceptions. If a handler cannot deal with a particular exception, it can rethrow the exception to the next handler in the stack. Exceptions propagate through the handler stack until it reaches the default handler which causes Parrot to exit.

Annotations

Annotations are pieces of metadata that can be stored in a bytecode file to give some information about what the original source code looked like. This is especially important when dealing with high-level languages. We'll go into detail about annotations and their use in CHP-10Chapter 10.

Annotations are created using the c<.annotation> keyword. Annotations consist of a key/value pair, where the key is a string and the value is an integer, a number, or a string. Since annotations are stored compactly as constants in the compiled bytecode, PMCs cannot be used.

  .annotation 'file', 'mysource.lang'
  .annotation 'line', 42
  .annotation 'compiletime', 0.3456

Annotations exist, or are "in force" throughout the entire compilation unit, or until they are redefined. Creating a new annotation with the same name as an old one overwrites it with the new value. The current hash of annotations can be retrieved with the annotations opcode:

  .annotation 'line', 1
  $P0 = annotations # {'line' => 1}
  .annotation 'line', 2
  $P0 = annotations # {'line' => 2}

Or, to retrieve a single annotation by name, you can write:

  $I0 = annotations 'line'

Annotations in Exceptions

Exception objects contain information about the annotations that were in force when the exception was thrown. These can be retrieved with the 'annotation'() method of the exception PMC object:

  $I0 = $P0.'annotations'('line')  # only the 'line' annotation
  $P1 = $P0.'annotations'()        # hash of all annotations

Exceptions can also give out a backtrace to try and follow where the program was exactly when the exception was thrown:

  $P1 = $P0.'backtrace'()

The backtrace PMC is an array of hashes. Each element in the array corresponds to a function in the current call stack. Each hash has two elements: 'annotation' which is the hash of annotations that were in effect at that point, and 'sub' which is the Sub PMC of that function.

cpanm Parrot::PMC

perl -MCPAN -e shell
install Parrot::PMC