[Haskell-cafe] type families and type signatures

Manuel M T Chakravarty chak at cse.unsw.edu.au
Fri Apr 11 00:24:26 EDT 2008

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

That's bad, I agree.

> 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.

Definitely.  Can you give an example?


> On Thu, Apr 10, 2008 at 5:52 AM, Manuel M T Chakravarty <chak at cse.unsw.edu.au 
> > 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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