[Haskell-cafe] Re: Why I Love Haskell In One Simple Example
mads_lindstroem at yahoo.dk
Mon Jun 27 16:12:38 EDT 2005
Hi John Goerzen
> On 2005-06-27, Mads Lindstrøm <mads_lindstroem at yahoo.dk> wrote:
> > Hi John
> >> test :: forall a. (Num a) => a
> >> test = 2 * 5 + 3
> [ snip ]
> > I had newer seen anybody use "forall a." in function signatures before,
> > and therefore was curious about its effect. This is probably do to my
> > inexperience regarding Haskell. However, I tried to remove it and wrote
> > this instead:
> If you omit it, the compiler will decide that test is some arbitrary
> type (Double, Integer, whatever). While rpnShow, etc. will still work,
> they will not show you the same thing, since the compiler will have
> already "optimized" the expression down to one set type.
They show the same thing on my computer, namely
"2 5 * 3 +"
if I do ":type test" I get (with or without the forall a.):
rpnShow :: Num a => SymbolicManip a -> String
> Which compiler or interpreter are you using?
-- Hugs Version November 2003
I have not tried with ghc(i), as it would not load MissingH.Str. Some
problem I will have to look into later.
> > I tried to find documentation about the use of the forall keyword in
> > respect to functions (I do know about it in with respect to
> > existentially quantified types), but with no luck. So, if anybody has
> > some good pointers, please let med know about it.
> Note that test in this example is not a function.
OK, I assumed it was, as I thought all functions started with lower case
and all modules, classes, and data/type constructors started with upper
case. It does not take any variables as input, but that is still a
function in my book (but I could be wrong there. I am no mathematician).
> -- John
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
note that I am not using:
test :: a -- this will actually not compile
or not giving any type signatur, but using:
test :: (Num a) => a
More information about the Haskell-Cafe