Add inferrability example (#3)

README.md

@@ -17,15 +17,17 @@ Features:
 ```julia
 julia> using Try
 
-julia> result = Try.getindex(Dict(:a => 111), :a);
+julia> result = Try.getindex(Dict(:a => 111), :a)
+Try.Ok: 111
 
 julia> Try.isok(result)
 true
 
 julia> Try.unwrap(result)
 111
 
-julia> result = Try.getindex(Dict(:a => 111), :b);
+julia> result = Try.getindex(Dict(:a => 111), :b)
+Try.Err: KeyError: key :b not found
 
 julia> Try.iserr(result)
 true
@@ -79,8 +81,30 @@ mymap(x -> x + 1, (x for x in 1:5 if isodd(x)))
 
 Try.jl provides an API inspired by Rust's `Result` type.  However, to fully
 unlock the power of Julia, Try.jl uses the *small `Union` types* instead of a
-concretely typed sum type.  Furthermore, it optionally supports concretely-typed
-returned value when `Union` is not appropriate.
+concretely typed `struct` type.  This is essential for idiomatic clean
+high-level Julia code that avoids computing output type manually.  However, all
+previous attempts in this space (such as
+[ErrorTypes.jl](https://github.com/jakobnissen/ErrorTypes.jl),
+[ResultTypes.jl](https://github.com/iamed2/ResultTypes.jl), and
+[Expect.jl](https://github.com/KristofferC/Expect.jl)) use a `struct` type for
+representing the result value (see
+[`ErrorTypes.Result`](https://github.com/jakobnissen/ErrorTypes.jl/blob/c3a7d529716ebfa3ee956049f77f606b6c00700b/src/ErrorTypes.jl#L45-L47),
+[`ResultTypes.Result`](https://github.com/iamed2/ResultTypes.jl/blob/42ebadf4d859964efa36ebccbeed3d5b65f3e9d9/src/ResultTypes.jl#L5-L8),
+and
+[`Expect.Expected`](https://github.com/KristofferC/Expect.jl/blob/6834049306c2b53c1666cbed504655e36b56e3b4/src/Expect.jl#L6-L9)).
+Using a concretely typed `struct` as returned type has some benefits in that it
+is easy to control the result of type inference.  However, this is at the cost
+of losing the opportunity for the compiler to eliminate the success and/or
+failure branches.  A similar optimization can happen in principle with the
+concrete `struct` approach with some aggressive (post-inference) inlining,
+scalar replacement of aggregate, and dead code elimination.  However, since type
+inference is the main driving force in the inter-procedural analysis of the
+Julia compiler, `Union` return type is likely to continue to be the most
+effective way to communicate the intent of the code with the compiler (e.g., if
+a function call always succeeds, return an `Ok{T}`).  (That said, Try.jl also
+contains supports for concretely-typed returned value when `Union` is not
+appropriate. This is for experimenting if such a manual "type-stabilization" is
+a viable approach and if providing a seamless interop API is possible.)
 
 A potential usability issue for using the `Result` type is that the detailed
 context of the error is lost by the time the user received an error.  This makes

examples/inferrability.jl

@@ -0,0 +1,14 @@
+module UnionTyped
+using Try
+g(xs) = Ok(xs)
+f(xs) = g(xs) |> Try.and_then(xs -> Try.getindex(xs, 1)) |> Try.ok
+end  # module UnionTyped
+
+module ConcretelyTyped
+using Try
+g(xs) = Try.ConcreteOk(xs)
+function trygetfirst(xs)::Try.ConcreteResult{eltype(xs),BoundsError}
+    Try.getindex(xs, 1)
+end
+f(xs) = g(xs) |> Try.and_then(trygetfirst) |> Try.ok
+end  # module

src/Try.jl

@@ -17,9 +17,11 @@ end
 
 const DynamicResult{T,E} = Union{Ok{T},Err{E}}
 
+function _ConcreteResult end
+
 struct ConcreteResult{T,E<:Exception} <: AbstractResult{T,E}
     value::DynamicResult{T,E}
-    ConcreteResult{T,E}(value::DynamicResult{T,E}) where {T,E<:Exception} = new{T,E}(value)
+    global _ConcreteResult(::Type{T}, ::Type{E}, value) where {T,E} = new{T,E}(value)
 end
 
 const ConcreteOk{T} = ConcreteResult{T,Union{}}
@@ -83,7 +85,8 @@ using ..Try:
     Err,
     Ok,
     Result,
-    Try
+    Try,
+    _ConcreteResult
 
 using Base.Meta: isexpr
 

src/base.jl

@@ -37,6 +37,12 @@ end
     return Ok(v)
 end
 
+@inline function Try.getindex(xs::Tuple, i::Integer):: Result
+    i < 1 && return Err(BoundsError(xs, i))
+    i > length(xs) && return Err(BoundsError(xs, i))
+    return Ok(xs[i])
+end
+
 struct NotFound end
 
 function Try.getindex(dict::AbstractDict, key)::Result

src/core.jl

@@ -23,26 +23,36 @@ end
 Base.convert(::Type{Ok{T}}, ok::Ok) where {T} = Ok{T}(ok.value)
 Base.convert(::Type{Err{E}}, err::Err) where {E} = Err{E}(err.value)
 
+_concrete(result::Ok) = _ConcreteResult(Try.oktype(result), Union{}, result)
+_concrete(result::Err) = _ConcreteResult(Union{}, Try.errtype(result), result)
+
+Try.ConcreteOk(value) = _concrete(Ok(value))
+Try.ConcreteOk{T}(value) where {T} = _concrete(Ok{T}(value))
+Try.ConcreteErr(value) = _concrete(Err(value))
+Try.ConcreteErr{E}(value) where {E} = _concrete(Err{E}(value))
+
+Base.convert(::Type{ConcreteResult{T,E}}, result::Ok) where {T,E} =
+    _ConcreteResult(T, E, convert(Ok{T}, result))
+Base.convert(::Type{ConcreteResult{T}}, result::Ok) where {T} =
+    _ConcreteResult(T, Union{}, convert(Ok{T}, result))
+
+Base.convert(::Type{ConcreteResult{T,E}}, result::Err) where {T,E} =
+    _ConcreteResult(T, E, convert(Err{E}, result))
+Base.convert(::Type{ConcreteResult{<:Any,E}}, result::Err) where {E} =
+    _ConcreteResult(Union{}, E, convert(Err{E}, result))
+
 function Base.convert(
     ::Type{ConcreteResult{T,E}},
     result::ConcreteResult{T′,E′},
 ) where {T,E,T′<:T,E′<:E}
     value = result.value
     if value isa Ok
-        return ConcreteResult{T,E}(Ok{T}(value.value))
+        return _ConcreteResult(T, E, Ok{T}(value.value))
     else
-        return ConcreteResult{T,E}(Err{E}(value.value))
+        return _ConcreteResult(T, E, Err{E}(value.value))
     end
 end
 
-Base.convert(::Type{ConcreteResult{T,E}}, result::ConcreteOk) where {T,E} =
-    ConcreteResult{T,E}(convert(Ok{T}, value.value))
-
-Base.convert(::Type{ConcreteResult{T,E}}, result::ConcreteErr) where {T,E} =
-    ConcreteResult{T,E}(convert(Err{E}, value.value))
-
-Try.ConcreteOk{T}(value) where {T} = Try.ConcreteOk{T}(Ok{T}(value))
-Try.ConcreteErr{E}(value) where {E} = Try.ConcreteErr{E}(Err{E}(value))
 Try.oktype(::Type{R}) where {T,R<:AbstractResult{T}} = T
 Try.oktype(result::AbstractResult) = Try.oktype(typeof(result))
 

test/TryTests/src/TryTests.jl

@@ -3,6 +3,7 @@ module TryTests
 include("test_base.jl")
 include("test_errortrace.jl")
 include("test_tools.jl")
+include("test_inferrability.jl")
 include("test_doctest.jl")
 
 end  # module TryTests

test/TryTests/src/test_inferrability.jl

@@ -0,0 +1,16 @@
+module TestInferrability
+
+using Test
+
+include("../../../examples/inferrability.jl")
+
+should_test_module() = lowercase(get(ENV, "JULIA_PKGEVAL", "false")) != "true"
+
+function test()
+    @test @inferred(UnionTyped.f((111,))) == Some(111)
+    @test @inferred(UnionTyped.f(())) === nothing
+    @test ConcretelyTyped.f((111,)) == Some(111)
+    @test ConcretelyTyped.f(()) === nothing
+end
+
+end  # module