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0 pointshexane3605y ago0 comments

MLStyle.jl is nice, but I think Julia would really benefit from having it enshrined in the language. Multiple dispatch is great for a lot of problems, but sometimes it makes more sense to do your pattern matching inline.

For tagged unions, the difference is in memory layout. A real tagged union type would eliminate indirection and allow more code to be type stable.

For instance,

    mutable struct NotTypeStable
        o::Union{Some{Int}, Nothing}
    end

    function not_type_stable()
        o = returns_option()
        if isnothing(o)
            0
        else
            1
        end
    end

versus:

    enum Option{T}
        Some(T)
        None
    end

    mutable struct TypeStable
        o::Option{Int}
    end

    function type_stable()
        o = returns_option()
        match o
            Some(_) => 1
            None => 0
        end
    end

Both of these features aren't strictly necessary, but they act as a compliment to Julia's existing dynamic mechanisms (multiple dispatch & Union types)

mutable struct NotTypeStable o::Union{Some{Int}, Nothing} end function not_type_stable() o = returns_option() if isnothing(o) 0 else 1 end end

enum Option{T} Some(T) None end mutable struct TypeStable o::Option{Int} end function type_stable() o = returns_option() match o Some(_) => 1 None => 0 end end

0 comments

5 comments · 2 top-level

eigenspace5y ago· 2 in thread

Here's me copying your second example as closely as possible in julia:

    using MLStyle

    mutable struct TypeStable
        o::Union{Some{Int}, Nothing}
    end

    MLStyle.@as_record TypeStable
    MLStyle.@as_record Nothing

    function type_stable()
        o = returns_option()
        @match o.o begin
            Some(_)   => 1
            Nothing() => 0
        end
    end

    returns_option() = TypeStable(rand(Bool) ? Some(rand(1:10)) : nothing)

And what does the compiler have to say about it?

    julia> Core.Compiler.return_type(type_stable, Tuple{})
    Int64

and

    julia> @code_warntype type_stable()
    Variables
      #self#::Core.Const(type_stable)
      o::TypeStable
      259::Union{Nothing, Some{Int64}}
      return#257::Union{Nothing, Int64}

    Body::Int64
    1 ─       (o = Main.returns_option())
    │         (return#257 = Main.nothing)
    │         (259 = Base.getproperty(o, :o))
    │   %4  = (259 isa Nothing)::Bool
    └──       goto #3 if not %4
    2 ─       (return#257 = 0)
    └──       goto #5
    3 ─ %8  = (259::Some{Int64} isa Some)::Core.Const(true)
    │         %8
    │   %10 = (259::Some{Int64} !== Main.nothing)::Core.Const(true)
    │         %10
    │         (return#257 = 1)
    └──       goto #5
    4 ─       Core.Const(:((error)("matching non-exhaustive, at #= REPL[9]:3 =#")))
    5 ┄       return return#257::Int64

This is on version 1.6.1 for me, but should work fine on earlier releases. I agree that having MLStyle.jl pattern matching bundled into julia would be great!

hexane360OP5y ago

Doesn't the TypeStable struct just hide the unknown type, not remove it? The type of o.o still needs to be looked up at runtime. Furthermore, TypeStable itself has to be on the heap, which I think means this causes two pointer lookups rather than just one.

eigenspace5y ago

> Doesn't the TypeStable struct just hide the unknown type, not remove it?

No, of you look at the typed IR I posted, it says

    259::Union{Nothing, Some{Int64}}

and

    (259 = Base.getproperty(o, :o))

So it's inferred properly to be a small union which julia handles efficiently at runtime:

    julia> @btime type_stable()
      18.988 ns (1 allocation: 32 bytes)
    1

> Furthermore, TypeStable itself has to be on the heap, which I think means this causes two pointer lookups rather than just one.

It's on the heap because you wrote

   mutable struct

in your example and I was just mimicking you as close as possible. If you don't actually want it to be mutable, then you can just remove the keyword mutable and it'll be even faster and have no heap allocations:

    julia> @btime type_stable()
     18.426 ns (0 allocations: 0 bytes)
    1

1 more reply

celrod5y ago· 1 in thread

Try Julia nightly (1.7):

  julia> @code_warntype not_type_stable()
  MethodInstance for not_type_stable()
    from not_type_stable() in Main at REPL[2]:1
  Arguments
    #self#::Core.Const(not_type_stable)
  Locals
    o::Any
  Body::Int64
  1 ─      (o = Main.returns_option())
  │   %2 = Main.isnothing(o)::Bool
  └──      goto #3 if not %2
  2 ─      return 0
  3 ─      return 1
  
  
  julia> versioninfo()
  Julia Version 1.7.0-DEV.1169
  Commit e5d7ef01b0* (2021-05-26 14:17 UTC)

Note `Body::Int64`. With older Julia versions, it should still be type stable if you did `o === nothing` instead of `isnothing(o)`.

hexane360OP5y ago

Right, I'm referring to the local o being Any, meaning that it must be behind a pointer lookup. I should have clarified because the more common usage is about the function's return value.

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0 comments

5 comments · 2 top-level

eigenspace5y ago· 2 in thread

Here's me copying your second example as closely as possible in julia:

    using MLStyle

    mutable struct TypeStable
        o::Union{Some{Int}, Nothing}
    end

    MLStyle.@as_record TypeStable
    MLStyle.@as_record Nothing

    function type_stable()
        o = returns_option()
        @match o.o begin
            Some(_)   => 1
            Nothing() => 0
        end
    end

    returns_option() = TypeStable(rand(Bool) ? Some(rand(1:10)) : nothing)

And what does the compiler have to say about it?

    julia> Core.Compiler.return_type(type_stable, Tuple{})
    Int64

and

    julia> @code_warntype type_stable()
    Variables
      #self#::Core.Const(type_stable)
      o::TypeStable
      259::Union{Nothing, Some{Int64}}
      return#257::Union{Nothing, Int64}

    Body::Int64
    1 ─       (o = Main.returns_option())
    │         (return#257 = Main.nothing)
    │         (259 = Base.getproperty(o, :o))
    │   %4  = (259 isa Nothing)::Bool
    └──       goto #3 if not %4
    2 ─       (return#257 = 0)
    └──       goto #5
    3 ─ %8  = (259::Some{Int64} isa Some)::Core.Const(true)
    │         %8
    │   %10 = (259::Some{Int64} !== Main.nothing)::Core.Const(true)
    │         %10
    │         (return#257 = 1)
    └──       goto #5
    4 ─       Core.Const(:((error)("matching non-exhaustive, at #= REPL[9]:3 =#")))
    5 ┄       return return#257::Int64

This is on version 1.6.1 for me, but should work fine on earlier releases. I agree that having MLStyle.jl pattern matching bundled into julia would be great!

hexane360OP5y ago

eigenspace5y ago

> Doesn't the TypeStable struct just hide the unknown type, not remove it?

No, of you look at the typed IR I posted, it says

    259::Union{Nothing, Some{Int64}}

and

    (259 = Base.getproperty(o, :o))

So it's inferred properly to be a small union which julia handles efficiently at runtime:

    julia> @btime type_stable()
      18.988 ns (1 allocation: 32 bytes)
    1

> Furthermore, TypeStable itself has to be on the heap, which I think means this causes two pointer lookups rather than just one.

It's on the heap because you wrote

   mutable struct

    julia> @btime type_stable()
     18.426 ns (0 allocations: 0 bytes)
    1

1 more reply

celrod5y ago· 1 in thread

Try Julia nightly (1.7):

  julia> @code_warntype not_type_stable()
  MethodInstance for not_type_stable()
    from not_type_stable() in Main at REPL[2]:1
  Arguments
    #self#::Core.Const(not_type_stable)
  Locals
    o::Any
  Body::Int64
  1 ─      (o = Main.returns_option())
  │   %2 = Main.isnothing(o)::Bool
  └──      goto #3 if not %2
  2 ─      return 0
  3 ─      return 1
  
  
  julia> versioninfo()
  Julia Version 1.7.0-DEV.1169
  Commit e5d7ef01b0* (2021-05-26 14:17 UTC)

Note `Body::Int64`. With older Julia versions, it should still be type stable if you did `o === nothing` instead of `isnothing(o)`.

hexane360OP5y ago

Right, I'm referring to the local o being Any, meaning that it must be behind a pointer lookup. I should have clarified because the more common usage is about the function's return value.

j / k navigate · click thread line to collapse