Re: [Haskell-cafe] Re: Monad Set via GADT

On Friday 12 January 2007 09:04, Simon Peyton-Jones wrote:

| > On 1/3/07, Roberto Zunino wrote: | >> 1) Why the first version did not typececk? | > | > 1) Class constraints can't be used on pattern matching. They ARE | > restrictive on construction, however. This is arguably bug in the | > Haskell standard. It is fixed in GHC HEAD for datatypes declared | > in the GADT way, so as not to break H98 code: | | http://article.gmane.org/gmane.comp.lang.haskell.cvs.all/29458/matc |h=gadt+class+context | | To quote from there: "I think this is stupid, but it's what H98 | says." | | Maybe it is time to consider it deprecated to follow the Haskell 98 | standard /to the letter/.

GHC follows this strange standard when you write data type decls in H98 form

data Eq a => T a = C a Int | D

Here, pattern-matching on either C or D will cause an (Eq a) constraint to be required.

However, GHC does *not* follow this convention when you write the data type decl in GADT style syntax:

data T a where C :: Eq a => a -> Int -> T a D :: T a

Here, (a) you can be selective; in this case, C has the context but D does not. And (b) GHC treats the constraints sensibly: - (Eq a) is *required* when C is used to construct a value - (Eq a) is *provided* when C is used in pattern matching

In short, in GADT-style syntax, GHC is free to do as it pleases, so it does the "right" thing. In this case, then, you can avoid the H98 "bug" by using GADT-style syntax.

All of this is documented in the user manual. If it's not clear, please help me improve it.

Crystal clear. My remark was meant merely as a general observation. Cheers Ben