Re: [Haskell-cafe] type families and type signatures

["Lennart Augustsson"]

I didn't say TF was the only broken feature in GHC.  But I want to see the broken ones removed, instead of new ones added. :)

In the current GHC there are even definitions that are perfecty usable, that cannot be given the type signature that that was inferred.

At work we have the warning for missing signatures enabled, and we turn warnings into errors.  We have to disbale this for certain definitions, because you cannot give them a signature.  I find that broken.

  -- Lennart

On Thu, Apr 10, 2008 at 5:52 AM, Manuel M T Chakravarty <chak@xxxxxxxxxxxxxxx> wrote:

Lennart Augustsson:

Let's look at this example from a higher level.

Haskell is a language which allows you to write type signatures for functions, and even encourages you to do it.
Sometimes you even have to do it.  Any language feature that stops me from writing a type signature is in my opinion broken.
TFs as implemented in currently implemented ghc stops me from writing type signatures.  They are thus, in my opinion, broken.

The problem of ambiguity is not at all restricted to TFs.  In fact, you need neither TFs nor FDs to get the exact same behaviour.  You don't even need MPTCs:

{-# LANGUAGE FlexibleContexts #-}
module Ambiguity where

class C a

bar :: C (a, b) => b -> b
bar = id

bar' :: C (a, b) => b -> b
bar' = bar

This gives us

/Users/chak/Code/haskell/Ambiguity.hs:10:7:
  Could not deduce (C (a, b)) from the context (C (a1, b))
    arising from a use of `bar'
                 at /Users/chak/Code/haskell/Ambiguity.hs:10:7-9
  Possible fix:
    add (C (a, b)) to the context of the type signature for `bar''
    or add an instance declaration for (C (a, b))
  In the _expression_: bar
  In the definition of `bar'': bar' = bar

So, we have this problem as soon as we have flexible contexts and/or MPTCs, independent of TFs and FDs.

Manuel

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