Traitor

Traitor, noun, possibly from old French or somewhere. Definition:

1. A person who double-crosses or otherwise does harm to a trusting party.

2. An uncommon (some even say made-up) term for a person or package which bestows traits on (or betrays) a programming language.

Overview

The Traitor.jl package is a naive (and potentially evil) attempt to create a convenient framework for trait-based dispatch in the Julia programming language. Traits provide an alternative to single-inheritance multiple-dispatch, which would allow for complex combinations of object properties to be tested at method dispatch.

Currently the package has basic functionality, supporting the features discussed in the following sections. Some obvious oversights include lack of support for default values and keyword arguments.

Warning: please have some fun using this package, but it might not yet be suitable for production code.

Our expectations of a traits type system

Trait types are organized into the type system. An abstract type defines the trait class, while subtypes of them define examples of that trait.

For example:

abstract Size

immutable Big <: Size; end
immutable Medium <: Size; end
immutable Small <: Size; end

Types are annotated with traits post-hoc by returning the appropriate trait type from the constructor of the abstract trait class given the data type in question. So we could have:

# Note: both input and output are types
Size(::Union{Type{Int32},Type{Int64}}) = Small
Size(::Type{BigInt}) = Big
Size(::Type{Int128}) = Medium

The @traitor macro

The @traitor macro is used to define a method which will be followed by a further trait-based dispatch. Currently, it suffers from several limitations. Firstly, the function must already exist (use function f end if necessary). Further, @traitor methods are not compatible with pre-existing standard methods, so it is best to keep in mind that @traitor can destructively overwrite existing definitions. Finally, it doesn't currently support default values or keyword arguments.

The traits to dispatch upon are given by a flexible extra set of :: operators. For a quick example, we allow for instance:

@traitor function howbig(x::Any::Big)
    "Huge!"
end

# The `Any` is optional
@traitor function howbig(x::::Medium)
    "So-so"
end

# One can combine standard dispatch and traits-based dispatch. In this case,
# standard multiple-dispatch occurs first, and `Traitor` then selects the most
# appropriate trait-based submethod.
@traitor function howbig(x::Integer::Small)
    "Teensy..."
end

Unions of traits via Union

Trait constraints can be relaxed by writing Union{}s of trait types of the same trait class. For example:

@traitor roughlyhowbig(x::::Big) = "Huge!"
@traitor roughlyhowbig(x::::Union{Medium,Small}) = "Smallish"

Such unions encapsulate a larger set of Julia objects than the individual traits.

Intersection of traits via Tuple

Traits of different trait classes can be combined using a Tuple{...}. This represents a smaller set of Julia objects - those which satisfy all of the traits simultaneously. For example,

abstract Odor
immutable Smelly <: Odor; end

@traitor describeyourself(::::Tuple{Big,Smelly}) = "I'm big and smelly"

Internally to Traitor, the most generic trait-methods are defined by the Tuple{} trait, which has no trait constraints and therefore may represent any object.

Computable traits

To our utter dismay, it turns out that traits can be computed. Watch out for this slippery code below:

"The `Treason` module implements stuff that Jeff and Stefan really won't like."
module Treason
using Traitor

abstract Mutability
immutable Immutable <: Mutability; end
immutable Mutable <: Mutability; end

@pure Mutability(T) = T.mutable ? Mutable : Immutable

"Boring, old definition..."
@traitor function map(f, x::::Mutable)
    out = similar(x)
    for i = 1:length(x)
        out[i] = f(x[i])
    end
    return out
end

"Cool, but please optimize me onto the stack, oh Julia compiler gods"
@traitor function map(f, x::::Immutable)
    out = Ref(similar(x)) # Get one on the heap
    for i = 1:length(x) # Mutate it on the heap
        out[i] = f(x[i]) # Elsewhere define setindex!(::RefValue{Tuple}), etc
    end
    return out.x # Copy it back to the stack
end

function setindex!{T <: Tuple}(x::RefValue{T}, val, i::Integer)
    # grab a pointer and set some memory (safely, please...)
end

end # module

Help wanted: the @betray macro

We would like to have a macro, say @betray, that would be able to "steal" pre-existing method definitions and make them compatible with @traitor methods as a default fallback. The current roadblock is that we don't know how to take the code corresponding to a Method (or LambdaInfo) and insert it into a new method definition (since the LambdaInfo is lowered IR and it is normal to define methods with top-level expression syntax).

Acknowledgements

This is joint work by Chris Foster (@c42f) and Andy Ferris (@andyferris). We would like to thank QANTAS for providing a small space with limited distractions for such a long time, so that we could prototype this work.