Am Mittwoch 02 September 2009 23:24:46 schrieb aditya siram:
Hi all, Recently I wrote a function that takes a unique identifier that I called 'id'. I then tried to apply the 'id' function to it and GHC did not like that. But it should.
For example in 'test' I have told the compiler that the id argument is an Int. So type inference should be able to determine the first 'id' in 'id id' couldn't possibly be an Int, but it complains. So I explicitly told the compiler the type of 'id' in test1 - this didn't work either. The final function 'test3' works as expected. Is there something I am not understand about the way type inference is supposed to work?
test :: Int -> Int test id = id id
test1 :: Int -> Int test1 id = idFunc id where idFunc :: a -> a idFunc = id
test2 :: Int -> Int test2 myid@id = id myid
test3 :: Int -> Int test3 id = Prelude.id id
thanks ... -deech
It's not type inference, it's scoping. By calling the function's parameter id, you shadow the name id, so within that scope each (unqualified) mention of the name id refers to the function's parameter. In test3, you use the qualified name Prelude.id, thus tell the compiler that it's not the parameter you want but the identity function.