Add showConstructor to convert union value constructor to Text

[darichey]

Fixes #1204

Adds a showConstructor keyword that can be used to convert a union value to Text by its constructor name. For example:

⊢ :paste
-- Entering multi-line mode. Press <Ctrl-D> to finish.
| let Foo = < A | B : Natural | C : Natural -> Text >
| let a = showConstructor Foo.A
| let b = showConstructor (Foo.B 3)
| let c = showConstructor (Foo.C (\(n : Natural) -> ""))
| in
| "${a}${b}${c}"

"ABC"

It also works on Optional values:

⊢ showConstructor (None Natural)

"None"

⊢ showConstructor (Some 0)

"Some"

I have a draft implementation for dhall-haskell: darichey/dhall-haskell@64ef9df

[Gabriella439]

Looks great! I think all this needs is tests

[MonoidMusician]

Looks good to me, just this one detail.

[sjakobi]

Looks like a great feature! :)

I've been trying to think of some "interesting" type-inference failure tests. So far I've only come up with:

• showConstructor < A : Bool >.A
• showConstructor None

Please also add type-inference and normalization success tests for something like showConstructor (< A : Bool>.A False).

I also wonder whether showConstructor ought to work for Bools. I don't see why not. Our main reason for not allowing merge for Bools doesn't seem to apply in this case.

[darichey]

Thanks for the test suggestions! I've added those.

showConstructor for Bools sounds reasonable, though I wonder what the use case would be where you wouldn't just use Bool/show.

[sjakobi]

showConstructor for Bools sounds reasonable, though I wonder what the use case would be where you wouldn't just use Bool/show.

It might be just slightly more convenient than importing Bool/show from the Prelude. Not a very important addition though.