[Haskell-cafe] Can someone explain this typing restriction?

[Bernard James POPE]

On Mon, Aug 14, 2006 at 02:17:05PM +0200, Tim Walkenhorst wrote:
> I've stumbled over the following (code *extremely* simplified):
> 
> f :: Bool
> f = g 1
> 
> g :: Num a => a -> Bool
> g _ = f
> 
> This results in the following error message (on GHC):
> 
>    Contexts differ in length
>    When matching the contexts of the signatures for
>      f :: Bool
>      g :: forall a. (Num a) => a -> Bool
>    The signature contexts in a mutually recursive group should all be 
> identical


> My first question is:
> 
> 1) Is there a name for this restriction: I can't clearly identify it as 
> some case of Let-Bound Polymorphism or the monomorphism restriction. 
> [http://www.cs.auckland.ac.nz/references/haskell/haskell-intro-html/pitfalls.html]

This probably doesn't answer everything, but it is discussed in the following
section of the Haskell Language Report:

   http://haskell.org/onlinereport/decls.html#sect4.5.2

A problem with the Report is that sometimes it tells you the rules, but it
doesn't always justify them with detailed comments. 

I had a feeling this was discussed in "Typing Haskell in Haskell" by Mark 
Jones, but after a quick skim I can't find it again (I thought it would be
in Section 11.6 of his paper). 

Cheers,
Bernie.