no longer converting Const to Identity

Author: schlichtanders

Changelog.md

@@ -6,12 +6,19 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [2.0.0] - ?????
 ### Added
 - `Base.get` and `Base.getindex` are now both implemented for `Const` (like they are implemented already for `Identity`), to simplify working with `Const`.
 
 ### Fixed
 - `Option(3)` works now
 - The constructors `Const{ElementType}(value)` and `Identity{ElementType}(value)` had been removed already, however were still used. Maybe this now gave errors because of newer julia version, don't know, but now everything uses `Const(value)` and `Identity(value)` instead.
+
+### Removed
+- `Option{T}(value::T)` didn't work and is removed now, as it does not provide any benefit anylonger, which is a breaking change
+- `Option{T}()` is also removed for completeness, which is a breaking change
+- `Const` no longer converts to `Identity`. This was added so that a `Const` nested within an `Identity` can be flattened. However, there is a better alternative, namely now the `flatmap` operator on `Identity` just strips away the outer Identity, without any call to `convert`. This is simpler and more convenient approach which also makes the hacky conversion no longer needed. Still this is a breaking change.
+
 ## [1.0.0] - 2020-07-19
 ### Added
 - CI via GitHubActions

src/Identity.jl

@@ -39,7 +39,8 @@ Base.iterate(a::Identity) = a.value, nothing
 Base.iterate(a::Identity, state) = state
 Base.foreach(f, a::Identity) = begin f(a.value); nothing; end
 Base.map(f, a::Identity) = Identity(f(a.value))
-Base.Iterators.flatten(a::Identity) = convert(Identity, a.value)
+# for convenience, Identity does not use convert, whatever monad is returned is valid, providing maximum flexibility.
+Base.Iterators.flatten(a::Identity) = a.value
 
 Base.convert(::Type{Identity{T}}, x::Identity{T}) where {T} = x
 Base.convert(::Type{Identity{T}}, x::Identity) where {T} = Identity(convert(T, x.value))

src/convert.jl

@@ -21,9 +21,6 @@ end
 # Identity
 # --------
 
-# Const are just passed through when asked to convert to identity
-# this is intended special behaviour in order for Identity + Const to work together as Option/Either
-Base.convert(::Type{Identity}, x::Const) = x
 # ContextManager is executed
 Base.convert(::Type{<:Identity}, x::ContextManager) = Identity(run(x))
 

test/convert.jl

@@ -9,8 +9,6 @@ using DataTypesBasic
 @test convert(Vector, Const(43)) == []
 @test convert(Vector, @ContextManager cont -> cont(2)) == [2]
 
-@test convert(Identity, Option()) == Option()
-@test convert(Identity, Const(:error)) == Const(:error)
 @test convert(Identity, @ContextManager cont -> cont(2)) == Identity(2)
 @test_throws MethodError convert(Identity, [1,2,3])