On Wednesday 29 June 2011, 16:55:50, Roelof Wobben wrote:

Hello,

I'm now following the book Programming in Haskel by Hutton. IM now reading the chapter about types.

But I don't understand this :

False :: Bool True :: Bool ¬ :: Bool ---> Bool

So False and True has the type Bool. That's clear.

I find out that ¬ makes the opposite of True and False. But why is it also the type Bool --> Bool

It's a function from Bool to Bool, that type is denoted by Bool -> Bool in Haskell (generally, "a -> b" is the type of functions taking an argument of type "a" and returning a value of type "b"; it's particularly interesting if "b" is itself a type of functions, e.g. "b = c -> d", then the function has type "a -> (c -> d)" or "a -> c -> d" [in that position, the parentheses are optional, since the function arrow associates to the right], and it can also be regarded as the type of functions taking two arguments, one of type "a", one of type "c" and returning a value of type "d"; now, "d" might again be a function type, ...). Note that normally, it's written "not", and not with a fancy unicode symbol.

Roelof