[Haskell-cafe] type variable is ambiguous in a non-injective type family

[Cosmia Fu]

In short, given the following definition

instance  Ambi WhatEver1 where
  type SubAmbi a = ()

instance  Ambi WhatEver2 where
  type SubAmbi a = ()

When you try to call `toBool ()`, GHC cannot decide which instance to
use, they are both valid candidates.

----

Cosmia Fu



On Sun, Jul 28, 2019 at 8:56 PM Henry Laxen <nadine.and.henry at pobox.com> wrote:
>
> Dear Cafe,
>
> I really don't understand what is going on here.  I've searched, but I don't
> really understand the links that I found. The closest I've come is:
>
> https://sulzmann.blogspot.com/2013/01/non-injective-type-functions-and.html
>
> but I don't understand it enough to see my way to a solution.  Here is a
> minimal example of what I'm trying to do:
>
> --------------------------------------------------------------------------------
>
> {-# LANGUAGE TypeFamilies #-}
> {-# LANGUAGE AllowAmbiguousTypes #-}
>
> module Ambiguous where
>
> class Ambi a where
>   type SubAmbi a :: *
>   toSubAmbi :: a -> SubAmbi a
>   toBool :: SubAmbi a -> Bool
>
> whatsWrong :: Ambi a => a -> IO ()
> whatsWrong a = do
>   let
>     s = toSubAmbi a
>     b = toBool s
>   if b then print "True" else print "False"
>
>
> {-
>
> src/Games/Ambiguous.hs:15:16: error: …
>     • Couldn't match expected type ‘SubAmbi a0’
>                   with actual type ‘SubAmbi a’
>       NB: ‘SubAmbi’ is a non-injective type family
>       The type variable ‘a0’ is ambiguous
>     • In the first argument of ‘toBool’, namely ‘s’
>       In the expression: toBool s
>       In an equation for ‘b’: b = toBool s
>     • Relevant bindings include
>         s :: SubAmbi a
>           (bound at /home/henry/nadineloveshenry/bin/nlh/src/Games/Ambiguous.hs:14:5)
>         a :: a
>           (bound at /home/henry/nadineloveshenry/bin/nlh/src/Games/Ambiguous.hs:12:12)
>         whatsWrong :: a -> IO ()
>           (bound at /home/henry/nadineloveshenry/bin/nlh/src/Games/Ambiguous.hs:12:1)
>    |
> Compilation failed.
>
> -}
> --------------------------------------------------------------------------------
>
> Could any of you Haskell sages please explain how to do this?
>
> Best wishes,
> Henry Laxen
>
> --
> Nadine and Henry Laxen   The rest is silence
> Villa Alta #6
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