++ed by:
MAJENSEN
Author image Arne Johannessen

NAME

Neo4j::Types - Common Neo4j type system

VERSION

version 1.00

SYNOPSIS

 # direct use
 $node = bless $data, 'Neo4j::Types::Node';
 
 # indirect use
 $node = bless $data, 'Local::Node';
 
 package Local::Node;
 use parent 'Neo4j::Types::Node';
 # override methods as required

DESCRIPTION

The packages in this distribution offer a Neo4j type system for Perl. Other distributions for the Neo4j ecosystem such as Neo4j::Bolt and Neo4j::Driver can (if they so choose) use these packages either directly or indirectly.

If several such distributions share the same representation of Neo4j values, sharing data between distributions becomes more efficient and users may have an easier time alternating between them.

Packages in this distribution primarily define methods. They do not currently make any particular assumptions about their internal data structures. This distribution offers default implementations of the methods it defines; these are designed to work with Neo4j::Bolt data structures. But inheritors (such as Local::Node in the synopsis example) are free to use any data structure they like, provided they override methods as required to not change the API.

The methods defined by this distribution are loosely modelled on the Neo4j Driver API. They don't match that API precisely because the official Neo4j drivers don't always use the exact same method names for their functionality, and the Neo4j Driver API Spec currently doesn't discuss these methods.

The module Neo4j::Types itself currently only contains documentation, but you can use it as a shortcut to make all modules that are included in this distribution available to you.

CYPHER TYPES

The Neo4j Cypher Manual mentions a variety of types. This section discusses typical ways to implement these in Perl.

Composite types

Composite types are:

  • List

  • Map (also known as Dictionary)

In Perl, these types match simple unblessed array and hash references very nicely.

Node, Relationship, Path

Neo4j structural types may be represented as:

Scalar types

Values of the following types can in principle be stored as a Perl scalar. However, Perl scalars by themselves cannot cleanly separate between all of these types. This can make it difficult to convert scalars back to Cypher types (for example for the use in Cypher statements parameters).

Number (Integer or Float)

Both Neo4j and Perl internally distinguish between integer numbers and floating-point numbers. Neo4j stores these as Java long and double, which both are signed 64-bit types. In Perl, their precision is whatever was used by the C compiler to build your Perl executable (usually 64-bit types as well on modern systems).

Both Neo4j and Perl will automatically convert integers to floats to calculate an expression if necessary (like for 1 + 0.5), so the distinction between integers and floats often doesn't matter. However, integers and floats are both just scalars in Perl, which may make it difficult to create a float with an integer value in Neo4j (for example, trying to store $a = 2.0 + 1 as a property may result in the integer 3 being stored in Neo4j).

perlnumber explains further details on type conversions in Perl. In particular, Perl will also try to automatically convert between strings and numbers, but Neo4j will not. This may have unintended consequences, as the following example demonstrates.

 $id = get_id_from_node($node);  # returns an integer
 say "The ID is $id.";           # silently turns $id into a string
 $node = get_node_by_id($id);    # fails: ID must be integer

This latter situation may be solved by using unary coercions.

 $string = "$number";
 $number = 0 + $string;

In the future, the Neo4j::Types distribution might be extended to offer ways to better handle the issues described in this section.

String

Perl scalars are a good match for Neo4j strings. However, in some situations, scalar strings may easily be confused with numbers or byte arrays in Perl.

Neo4j strings are always encoded in UTF-8. Perl supports this as well (though string scalars that only contain ASCII are usually not treated as UTF-8 internally for efficiency reasons).

Boolean

Perl does not have a native boolean data type. It's trivial to map from Cypher booleans to truthy or non-truthy Perl scalars, but the reverse is difficult without additional information.

There are a multitude of modules on CPAN that try to solve this problem, including boolean, Types::Bool, and Types::Serialiser. Among them, JSON::PP::Boolean has the advantage that it has long been in Perl CORE.

Null

The Cypher null value can be neatly implemented as Perl undef.

Byte array

Byte arrays are not actually Cypher types, but still have some limited support as pass-through values in Neo4j. In Perl, byte arrays are most efficiently represented as string scalars with their UTF8 flag turned off (though there may be some gotchas; see "Working with SVs" in perlguts for details).

However, it usually isn't possible to determine whether such a scalar actually is supposed to be a byte array or a string; see "How can I recognise a UTF-8 string?" in perlguts. In the future, the Neo4j::Types distribution might be extended to offer ways to handle this.

Spatial types

The only spatial type currently offered by Neo4j is the point. It may be represented as Neo4j::Types::Point.

It might be possible to (crudely) represent other spatial types by using a list of points plus external metadata, or in a Neo4j graph by treating the graph itself as a spatial representation.

The coordinate reference systems of spatial points in Neo4j are currently severely constrained. There is no way to tag points with the CRS they actually use, and for geographic coordinates (lat/lon), only a single, subtly non-standard CRS is even supported. For uses that don't require the spatial functions that Neo4j offers, it might be best to eschew the point type completely and store coordinate pairs as a simple list in the Neo4j database instead.

Temporal types

Cypher temporal types include: Date, Time, LocalTime, DateTime, LocalDateTime, and Duration.

This distribution currently does not handle dates, times, or durations. It is suggested to use the existing packages DateTime and DateTime::Duration.

SEE ALSO

AUTHOR

Arne Johannessen <ajnn@cpan.org>

COPYRIGHT AND LICENSE

This software is Copyright (c) 2021 by Arne Johannessen.

This is free software, licensed under:

  The Artistic License 2.0 (GPL Compatible)