Il Sat, Aug 26, 2006 at 06:00:02PM +0400, Bulat Ziganshin ebbe a scrivere:
type HashFunction = (String->Int) ... createHash :: HashFunction -> IO Hash and createHash :: (String->Int) -> IO Hash
are equivalent. So, technically speaking, you can't declare function types, there is only one predefined type constructor that can construct any function type you need. But from practical POV, you can define synonyms for any function type you've constructed using '->'.
this possibility (POV) lead me to think it was possible to have a type constructor that could construct a function type.
Next point is that in Haskell functions are first-class values, i.e. you can hold them in data structures, pass as parameters to other functions and even return them as results. For example, the following function accepts function of type String->Int->Int and returns function of type (Int->Int):
func :: (String->Int->Int) -> (Int->Int) func f = f ""
ok, I've got it.
Of course, newtype can also be used to define type whose only field contains some function:
newtype F = F (String->Int)
Again, in this case language guarantees that internal F representation will be the same as for plain (Int->String) function and therefore the same as for type synonym:
type S = String->Int
But from programmer POV, the difference still holds: while S denotes function of given type and can be used interchangeably with full notion of this function type, F is just the type which internally contains such function.
As I said in a previous message it took me almost two day to grasp this distinction.
hope that it helps :)
sure it does! thank you very mu! andrea