Re: [Haskell-cafe] Re: Why I Love Haskell In One Simple Example

Hi John Goerzen

On 2005-06-27, Mads Lindstrøm wrote:

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Hi John

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test :: forall a. (Num a) => a test = 2 * 5 + 3

[ snip ]

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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 -98 :version -- 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.

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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

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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 /Mads Lindstrøm