[Hat] Qualified variable in pattern: T.mkRoot

[Ian Lynagh]

On Wed, Jun 29, 2005 at 12:11:22PM +0100, Thomas Davie wrote:
> 
> On 28 Jun 2005, at 21:03, Ken T Takusagawa wrote:
> 
> >The following short program:
> >
> >module Main where
> >Just foo = undefined
> >
> >Causes an error message:
> >hmake -hat hatd.hs
> >hat-trans   hatd.hs
> >Wrote Hat/hatd.hs
> >/usr/bin/haskell-compiler      -c -package hat -o Hat/hatd.o
> >Hat/hatd.hs
> >
> >Hat/hatd.hs:10:10: Qualified variable in pattern: T.mkRoot
> >
> >with hat 2.0.4 and ghc 6.4-2 (debian)
> 
> My last reply I realise was somewhat useless - in that the addition  
> of a main function still gives the error.  However, I think that this  
> is actually hat catching a syntactic error that the normal compilers  
> don't catch... The haskell syntax states:
> 
> decl -> gendecl
>      | (funlhs | pat0) rhs
> 
> where, gendecl covers type definitions, funlhs covers functions (and  
> must start with a lower case character), and pat0 covers 0 arity  
> paterns.

The 0 is the tightness of binding, not the arity.

pat^0 -> pat^1 -> ... -> pat^10 -> gcon apat_1

Such contructs look a bit odd at the top level, but I think

    foo x = f y
        where Just y = x

is more common. Even more so is

    [ x | Just x <- xs]


Thanks
Ian