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

28 Jul 2019

      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

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