Syntactic sugar for defining multiple parametric functions · helpdesk (published)

Stream: helpdesk (published)

Topic: Syntactic sugar for defining multiple parametric functions

Jesper Stemann Andersen (Nov 08 2021 at 13:19):

Hi,

The following is not possible:

let T <: Real
       foo(t::T) = t + t
       bar(t::T) = t - t
       baz(t::T) = t * t
end

Is there a way to avoid writing the same type parameter restrictions again and again for each function?

Fredrik Ekre (Nov 08 2021 at 13:58):

julia> let X = T where T <: Real
           global f(x::X) = x
           global g(x::X) = x
       end
g (generic function with 1 method)

julia> f(1)
1

julia> g(2.0)
2.0

Jesper Stemann Andersen (Nov 08 2021 at 14:16):

Thanks! :-)

Philipp Gabler (Nov 08 2021 at 14:18):

Fredrik Ekre said:

julia> let X = T where T <: Real

Which in this case is equivalent to let X = Real.

Jesper Stemann Andersen (Nov 22 2021 at 08:58):

So... how to define blocks with sets of dependent parametric types - e.g. (T1, T2) where {T1 <: Number,T2 <: AbstractVector{T1}}?

julia> let T = T1 where {T1 <: Number}, TVector = T2 where {T2 <: AbstractVector{T}}
       global function foo(x::T)
           @show typeof(x)
       end
       global function foo(v::TVector)
           @show typeof(v)
       end
       end
       foo(2)
       foo(2.0)
       foo(vec([1 2 3]))
typeof(x) = Int64
typeof(x) = Float64
ERROR: MethodError: no method matching foo(::Vector{Int64})
Closest candidates are:
  foo(::Number) at REPL[36]:3
  foo(::AbstractVector{Number}) at REPL[36]:6

Letting TVector = AbstractVector (without the parameterization on T), and defining foo(v::TVector{T}) also does not define foo for ::Vector{Int64}.

Fredrik Ekre (Nov 22 2021 at 09:01):

Note that

julia> Vector{Int} <: Vector{Real}
false

julia> Vector{Int} <: Vector{<:Real}
true

see https://docs.julialang.org/en/v1/manual/types/#man-parametric-composite-types

Jesper Stemann Andersen (Nov 22 2021 at 09:07):

Let me answer myself...

This works at least (the generic function must have a where T):

let T = Number, TVector = AbstractVector
    global function foo(x::T)
        @show typeof(x)
    end
    global function foo(v::TVector{T}) where T
        @show typeof(v)
    end
end

foo(2)
foo(2.0)
foo(vec([1 2 3]))

Jesper Stemann Andersen (Nov 22 2021 at 09:25):

... but all I achieved for AbstractVector/TVector was temporarily renaming it...

Sukera (Nov 22 2021 at 09:27):

parametric types in julia are invariant (except tuples)

Sukera (Nov 22 2021 at 09:27):

what would you like to achieve?

Sukera (Nov 22 2021 at 09:28):

if you want to define a function that takes vectors with elements of any number type, you can define foo(v::AbstractVector{<: Number})

Jesper Stemann Andersen (Nov 22 2021 at 09:30):

I would just like to avoid writing the same where {T1 <: AbstractT1, T2 <: AbstractT2{T1}} over and over for a block of methods.

Sukera (Nov 22 2021 at 09:31):

do you actually require T2 in the method?

Sukera (Nov 22 2021 at 09:31):

if not, the way I've written it above is equvialent

Jesper Stemann Andersen (Nov 22 2021 at 09:36):

Yes, I think so - see for a very simple example the two functions for SerializationTypeSerializer: https://github.com/IHPSystems/TypeSerializers.jl/blob/main/src/TypeSerializers.jl

Sukera (Nov 22 2021 at 09:37):

those don't use TSerializer in the method itself though, only T

Sukera (Nov 22 2021 at 09:37):

in which case foo(s::SerializationTypeSerializer{T})::T where T should be the same

Sukera (Nov 22 2021 at 09:39):

or more concrete for your example ::Type{<:SerializationTypeSerializer{T}}, I think

Jesper Stemann Andersen (Nov 22 2021 at 09:39):

This is what got me started (using those "type serializers"):
https://github.com/IHPSystems/SerializingChannels.jl/blob/main/src/SerializingChannels.jl

Sukera (Nov 22 2021 at 09:40):

(though intuitively, to me that already reads like relying a lot of type hierarchies, complicating things)

Sukera (Nov 22 2021 at 09:41):

if you're already restricting the type at construction, why check for the same requirement on method definition as well?

Sukera (Nov 22 2021 at 09:43):

e.g., the struct and the first function should be equivalent to this

struct SerializingChannel{T, TChannel <: AbstractChannel{Vector{UInt8}}, <: AbstractTypeSerializer{T}}
    channel::TChannel
end

function Base.put!(channel::SerializingChannel{T, _, TSerializer}, value::T) where {T, TSerializer}
    stream = serialize(TSerializer, value)
    data = take!(stream)
    put!(channel.channel, data)
end

if I'm not mistaken

Sukera (Nov 22 2021 at 09:43):

you don't have to give things a name if you're not going to use them in the function

Jesper Stemann Andersen (Nov 22 2021 at 09:47):

Ah yes - I see. Thanks a lot!

Sukera (Nov 22 2021 at 09:50):

you're welcome!

Sukera (Nov 22 2021 at 09:51):

also, if you want to avoid allocations, you may want to pass in an IO object instead of constructing one in serialize, only for it to be destroyed immediately again :)

Sukera (Nov 22 2021 at 09:51):

though that may be harder to work into your current design, since you have a channel of Vector{UInt8}

Sukera (Nov 22 2021 at 09:51):

(I'm also only speculating here, so may not be applicable to your situation)

Jesper Stemann Andersen (Nov 22 2021 at 10:05):

But I guess the programmer is still left to repetitive type constraints in the general case? I.e. where the type parameterization include abstract parametric types - like in the Number, AbstractVector{<:Number} example above.

Sukera (Nov 22 2021 at 10:07):

you mean across methods?

Jesper Stemann Andersen (Nov 22 2021 at 10:08):

Sukera said:

also, if you want to avoid allocations, you may want to pass in an IO object instead of constructing one in serialize, only for it to be destroyed immediately again :)

Yeah - might be possible. It's highly targeted on being able to use FlatBuffers for serialization and Zmq as a transport.

It was also a bit of a struggle to make it play nice with the Serialization module...

Sukera (Nov 22 2021 at 10:09):

julia> const T = Number
Number

julia> const TVec = AbstractVector{<:T}
AbstractVector{<:Number} (alias for AbstractArray{<:Number, 1})

julia> foo(v::TVec) = "nope"
foo (generic function with 1 method)

julia> foo(Int[])
"nope"

Sukera (Nov 22 2021 at 10:09):

though that only works when you don't have interdependencies between two types in a signature

Sukera (Nov 22 2021 at 10:10):

so if you require one type parameter of one type and another of another type to be the same, you _will_ need that where

Sukera (Nov 22 2021 at 10:10):

there's no way to express two different types having the same "slot" without specifying in which method that should apply

Sukera (Nov 22 2021 at 10:11):

(i.e. there's no real dependent typing in julia)

Sukera (Nov 22 2021 at 10:12):

also note that those const T are really only constant aliases - they're _not_ their own "versions" of that type

Sukera (Nov 22 2021 at 10:13):

It's highly targeted on being able to use FlatBuffers for serialization

well right now our serialize is always allocating an IOBuffer with its corresponding internal Vector{UInt8}, right?

Sukera (Nov 22 2021 at 10:13):

so my guess would be that it would be more efficient to directly write to the FlatBuffer instead of having those intermediary stores

Jesper Stemann Andersen (Nov 22 2021 at 10:19):

Sukera said:

julia> const T = Number
Number

julia> const TVec = AbstractVector{<:T}
AbstractVector{<:Number} (alias for AbstractArray{<:Number, 1})

julia> foo(v::TVec) = "nope"
foo (generic function with 1 method)

julia> foo(Int[])
"nope"

Excellent. A bit puzzled though, that I did not manage to make those T, TVec definitions work in a let block...
Should they just be nested let blocks? And how then to global'ize the method definitions?

Fredrik Ekre (Nov 22 2021 at 10:21):

Why doesn't it work with let?

julia> let T = Number, TVec = AbstractVector{<:T}
           global foo(::TVec) = "nope"
       end
foo (generic function with 1 method)

julia> foo(Int[])
"nope"

Jesper Stemann Andersen (Nov 22 2021 at 10:27):

Ah - it was probably just the parametric type invariance that I messed up (defining TVec = AbstractVector{T} instead of TVec = AbstractVector{<:T}).

Thanks again :-)

Last updated: Oct 02 2023 at 04:34 UTC