[Haskell-cafe] RebindableSyntax, ifThenElse, and pattern matching

[Adam Bergmark]

There are a number of things one might expect RebindableSyntax to support,
but it doesn't. Another examples is that `[1..2]' uses
`GHC.Enum.enumFromTo' instead of the `enumFromTo' in scope.

My guess is that RebindableSyntax isn't on the priority list.

- Adam



On Sun, Feb 7, 2016 at 8:08 PM, Chris Smith <cdsmith at gmail.com> wrote:

> In a custom prelude, I have written the following definitions:
>
> data Truth = True | False
>
> ifThenElse :: Truth -> a -> a -> a
> ifThenElse True x _ = x
> ifThenElse False _ x = x
>
> (==) :: a -> a -> Truth
> (==) = ...
>
> I'm replacing Bool with my own Truth type.  Clients will be built with
> RebindableSyntax, so that they will use these definitions for desugaring.
>
> However, if I write this:
>
> f :: Text -> Text
> f "r" = "rrr"
> f other = other
>
> I get a build error indicating that GHC expected the (==) operator to
> return a value of type Bool, rather than Truth.
>
> Shouldn't pattern matching desugar to use overloaded ifThenElse in these
> situations?  Or is it expected that use of GHC.Types.Bool is hard-coded
> even with RebindableSyntax enabled?
>
> Thanks,
> Chris
>
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