Add convert to Option from Option

Author: jakobnissen

src/ErrorTypes.jl

@@ -1,3 +1,12 @@
+# Note: This module is full of code like:
+# if x.data isa Thing
+#     x.data._1
+# else
+#
+# this is so the compiler won't typecheck inside the if blocks.
+# that in turns leads to more effective code.
+
+
 module ErrorTypes
 
 using SumTypes
@@ -102,6 +111,24 @@ None
 const none = None{Nothing}().data
 Base.convert(::Type{<:Option{T}}, ::None{Nothing}) where T = None{T}()
 
+function Base.convert(::Type{Option{T1}}, x::Option{T2}) where {T1, T2 <: T1}
+    data = x.data
+    if data isa Thing
+        Thing{T1}(data._1)
+    else
+        None{T1}()
+    end
+end
+
+function Base.convert(::Type{Option{T1}}, x::Option{T2}) where {T1, T2}
+    data = x.data
+    if data isa None
+        return None{T1}()
+    else
+        error("cannot convert Thing value")
+    end
+end
+
 """
     Thing(x::T)
 

test/runtests.jl

@@ -8,13 +8,19 @@ using Test
     @test !(None{Float32}() isa Option{AbstractFloat})
     @test !(None{Int}() isa None)
 
-    # Conversion
+    # Conversion from none
     @test convert(Option{Int}, none) isa Option{Int}
     @test convert(Option{Some{Nothing}}, none) isa Option{Some{Nothing}}
-    
     @test_throws MethodError convert(Int, none)
     @test_throws MethodError convert(Option{String}, "foo")
 
+    # Convert from Option
+    @test convert(Option{Integer}, Thing(1)) isa Option{Integer}
+    @test convert(Option{Union{String, Dict}}, Thing("foo")) isa Option{Union{String, Dict}}
+    @test convert(Option{String}, None{Int}()) isa Option{String}
+    @test convert(Option{Float64}, None{Float64}()) isa Option{Float64}
+    @test_throws ErrorException convert(Option{String}, Thing(1))
+
     # is_none
     @test is_none(None{Int}())
     @test is_none(convert(Option{String}, none))