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)`.

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.

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!

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.

> 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

>small union which julia handles efficiently at runtime

Oh! I didn't realize Julia already optimized small unions.

https://julialang.org/blog/2018/08/union-splitting/ https://docs.julialang.org/en/v1/devdocs/isbitsunionarrays/

I wasn't quite sure whether something like this was possible given the semantics of Julia.

Personally, I'd still prefer something more explicit, so it would be more obvious when a union is being handled efficiently. But given that small unions are optimized, I can see why Julia would make the opposite decision.

This makes me feel a lot better about writing Julia code with union types. Thanks

Yep, small union optimizations are a godsend!