[Haskell-cafe] Hugs vs. GHCi

[Daniel Fischer]

Am Freitag 29 Mai 2009 20:35:47 schrieb Dan Weston:
> http://haskell.org/onlinereport/exps.html#sect3.12
>
> "Pattern bindings are matched lazily; an implicit ~ makes these patterns
> irrefutable. For example,
>
> let (x,y) = undefined in e
>
> does not cause an execution-time error until x or y is evaluated."
>
> So GHCi is correct.
>
> Dan

It's not the matching that prdouces the error,

Haskell 98 mode: Restart with command line option -98 to enable extensions

Type :? for help
Hugs> let f x = let g y = [x,y] in (g 1, g []) in 1
ERROR - Cannot infer instance
*** Instance   : Num [a]
*** Expression : f

Hugs> :q
[Leaving Hugs]

dafis at linux-mkk1:~/Haskell/CafeTesting> hugs -98

Hugs mode: Restart with command line option +98 for Haskell 98 mode

Type :? for help
Hugs> let f x = let g y = [x,y] in (g 1, g []) in 1
1
Hugs> :t \x -> let g y = [x,y] in (g 1, g [])
\x -> let {...} in (g 1,g []) :: Num [a] => [a] -> ([[a]],[[a]])
Hugs>

In Haskell98 mode, hugs looks for an 
instance Num [a]
eagerly, doesn't find one and complains, while ghci is content with inferring a legal 
(though uninstantiated) type for the unused expression. In hugs mode, hugs does the same.

I don't know if the standard requires or disallows either behaviour. If it does, one of 
the two is wrong, if it's left to the implementation, both are right.

>
> Vladimir Reshetnikov wrote:
> > Hi,
> >
> > The following expression evaluates to 1 in GHCi, but results in an
> > error in Hugs:
> >
> > let f x = let g y = [x,y] in (g 1, g []) in 1
> >
> > What is the correct behavior?
> >
> > Thanks
> > Vladimir