The reason a gets a single type is the monomorphism restriction (read the report). Using NoMonomorphismRestriction your example with a works fine.
On Thu, Oct 8, 2009 at 12:29 PM, Cristiano Paris <fr...@theshire.org> wrote: > On Thu, Oct 8, 2009 at 11:04 AM, minh thu <not...@gmail.com> wrote: >> Hi, >> >> I'd like to know what are the typing rules used in Haskell (98 is ok). >> >> Specifically, I'd like to know what makes >> >> let i = \x -> x in (i True, i 1) >> >> legal, and not >> >> let a = 1 in (a + (1 :: Int), a + (1.0 :: Float)) >> >> Is it correct that polymorphic functions can be used polymorphically >> (in multiple places) while non-functions receive a monomorphic type ? > > First, "1" IS a constant function so it's in no way special and is a > value like any other. > > That said, the type of 1 is (Num t) => t, hence polymorphic. But, when > used in the first element of the tuple, a is assigned a more concrete > type (Int) which mismatches with the second element of the tuple, > which is a Float. > > If you swap the tuple, you'll find that the error reported by ghci is > the very same as before, except that the two types are swapped. > > Is it possible to rewrite the expression so as to work? The answer is > yes, using existential quantification (and Rank2 polymorphism). > > Here you are: > {-# LANGUAGE ExistentialQuantification, Rank2Types #-} > foo :: (forall a. (Num a) => a) -> (Int,Float) > foo = \x -> (x + (1 :: Int), x + (1 :: Float)) > > Hence: > > foo 1 --> (2,2.0) > > Bye, > > Cristiano > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe