From 1049d50e6e8f680e4961fcb14d21004d127f691e Mon Sep 17 00:00:00 2001
From: Takafumi Arakaki <aka.tkf@gmail.com>
Date: Sun, 24 Apr 2022 02:54:28 -0400
Subject: [PATCH] Add `Try.transpose`

---
 docs/src/index.md               |  1 +
 src/Try.jl                      |  1 +
 src/docs/transpose.md           | 17 +++++++++++++++++
 src/tools.jl                    | 21 +++++++++++++++++++++
 test/TryTests/src/test_tools.jl | 15 +++++++++++++++
 5 files changed, 55 insertions(+)
 create mode 100644 src/docs/transpose.md

diff --git a/docs/src/index.md b/docs/src/index.md
index 6152542..610549f 100644
--- a/docs/src/index.md
+++ b/docs/src/index.md
@@ -16,6 +16,7 @@ Try.unwrap_err
 Try.oktype
 Try.errtype
 Try.map
+Try.transpose
 ```
 
 ## Short-circuit evaluation
diff --git a/src/Try.jl b/src/Try.jl
index 875db05..54e6804 100644
--- a/src/Try.jl
+++ b/src/Try.jl
@@ -35,6 +35,7 @@ struct IsOkError <: InternalPrelude.Exception
 end
 
 function map end
+function transpose end
 
 macro and_return end
 function var"@?" end
diff --git a/src/docs/transpose.md b/src/docs/transpose.md
new file mode 100644
index 0000000..326b4b1
--- /dev/null
+++ b/src/docs/transpose.md
@@ -0,0 +1,17 @@
+    Try.transpose(result_of_maybe) -> maybe_of_result
+    Try.transpose(maybe_of_result) -> result_of_maybe
+
+Transpose `Union{Ok,Err}` wrapped in `Union{Some,Nothing}` and vice versa.
+
+# Extended help
+## Examples
+```julia
+using Try
+
+@assert Try.transpose(Ok(nothing)) === nothing
+@assert Try.transpose(Ok(Some(1))) === Some(Ok(1))
+@assert Try.transpose(Err(:error)) === Some(Err(:error))
+@assert Try.transpose(Some(Ok(1))) === Try.Ok(Some(1))
+@assert Try.transpose(Some(Err(:error))) === Try.Err(:error)
+@assert Try.transpose(nothing) === Try.Ok()
+```
diff --git a/src/tools.jl b/src/tools.jl
index b5a86ca..66794f2 100644
--- a/src/tools.jl
+++ b/src/tools.jl
@@ -21,3 +21,24 @@ TryMap(::Type{T}) where {T} = TryMap{Type{T}}(T)
 (f::TryMap)(x) = Try.map(f.f, x)
 
 Try.map(f::F) where {F} = TryMap(f)
+
+function Try.transpose(ok::Ok)
+    maybe = Try.unwrap(ok)::Union{Some,Nothing}
+    if maybe === nothing
+        nothing
+    else
+        Some(Ok(something(maybe)))
+    end
+end
+Try.transpose(err::Err) = Some(err)
+
+Try.transpose(::Nothing) = Ok()
+function Try.transpose(some::Some)
+    result = something(some)
+    if Try.isok(result)
+        Ok(Some(Try.unwrap(result)))
+    else
+        # Need to "forget" about any Ok value type information:
+        Err(Try.unwrap_err(result))
+    end
+end
diff --git a/test/TryTests/src/test_tools.jl b/test/TryTests/src/test_tools.jl
index dcf1989..9b8b2bd 100644
--- a/test/TryTests/src/test_tools.jl
+++ b/test/TryTests/src/test_tools.jl
@@ -151,4 +151,19 @@ function test_map()
     @test Try.map(x -> x + 1, nothing) === nothing
 end
 
+function test_double_transpose()
+    @testset for x in [
+        # Result-of-Maybe
+        Ok(Some(1)),
+        Ok(nothing),
+        Err(:error),
+        # Maybe-of-Result
+        Some(Ok(1)),
+        Some(Err(:error)),
+        nothing,
+    ]
+        @test Try.transpose(Try.transpose(x)) == x
+    end
+end
+
 end  # module