Optional: conversion from other AbstractVector (#25)

Author: nsajko

src/ParseUnparse.jl

@@ -44,6 +44,30 @@ module ParseUnparse
         function Base.iterate(::Optional, ::Any)
             nothing
         end
+        function convert_from_other_vector_eltype(::Type{Optional{T}}, ::Type) where {T}
+            T
+        end
+        function convert_from_other_vector_eltype(::Type{Optional}, ::Type{T}) where {T}
+            T
+        end
+        function convert_from_other_vector(::Type{O}, vec::AbstractVector) where {O <: Optional}
+            if 1 < length(vec)
+                throw(ArgumentError("got vector with more than one element, too long"))
+            end
+            T = convert_from_other_vector_eltype(O, eltype(vec))
+            Op = Optional{T}
+            if isempty(vec)
+                Op()
+            else
+                Op(only(vec))
+            end
+        end
+        function Base.convert(::Type{Optional{T}}, vec::AbstractVector) where {T}
+            convert_from_other_vector(Optional{T}, vec)
+        end
+        function Base.convert(::Type{Optional}, vec::AbstractVector)
+            convert_from_other_vector(Optional, vec)
+        end
     end
     """
         ContextFreeGrammarUtil::Module

test/runtests_optionals.jl

@@ -19,4 +19,12 @@ using Test
     @test [3] == collect(Float32, Optional(3))
     @test_throws BoundsError Optional(3.0)[2]
     @test_throws BoundsError Optional{Int}()[1]
+    @test (@inferred convert(Optional, Float32[])) isa Optional{Float32}
+    @test isempty(convert(Optional, Float32[]))
+    @test Optional(3) === convert(Optional, [3])
+    @test (@inferred convert(Optional{Float64}, Float32[])) isa Optional{Float64}
+    @test isempty(convert(Optional{Float64}, Float32[]))
+    @test Optional(3.0) === convert(Optional{Float64}, [3])
+    @test_throws ArgumentError convert(Optional, [3, 7])
+    @test_throws ArgumentError convert(Optional{Float64}, [3, 7])
 end