On Sun, Oct 25, 2009 at 1:37 PM, Isaac Dupree <ml@isaac.cedarswampstudios.org> wrote:
David Menendez wrote:
The expected type is what the context wants (it's *ex*ternal). The inferred type is what the expression itself has (it's *in*ternal).
So inferring the type Maybe () for bar seems wrong.
well, maybe GHC just gets it wrong enough of the time, that I got confused.
Or maybe ... When there are bound variables interacting, on the inside and outside, it gets confusing.
ghci: Prelude> \x -> (3+x) + (length x)
<interactive>:1:15: Couldn't match expected type `[a]' against inferred type `Int' In the second argument of `(+)', namely `(length x)' In the expression: (3 + x) + (length x) In the expression: \ x -> (3 + x) + (length x)
Your explanation of "expected" and "inferred" could make sense to me if the error message followed the "Couldn't match" line with, instead, "In the first argument of `length', namely `x'" because 'length' gives the context of expected list-type, but we've found out from elsewhere (a vague word) that 'x' needs to have type Int.
This had me confused for a while, but I think I've worked out what's happening. (+) is polymorphic, and GHC is giving it the type [a] -> [a] -> [a]. So the context is expecting [a], but we infer length x :: Int from the definition of length. In the alternate case, \x -> length x + (3+x), GHC gives the outer (+) the type Int -> Int -> Int, and the inner (+) the type [a] -> [a] -> [a], which is why we get the type mismatch complaint for 3+x instead of x. Note what happens if we use a monomorphic operator: Prelude> let (<>) = undefined :: Int -> Int -> Int Prelude> \x -> (3+x) <> length x <interactive>:1:22: Couldn't match expected type `[a]' against inferred type `Int' In the first argument of `length', namely `x' In the second argument of `(<>)', namely `length x' In the expression: (3 + x) <> length x Prelude> \x -> (3+x) + length x Here, GHC has concluded that x must be an Int, and thus can't be passed to length. Prelude> \x -> length x <> (3+x) <interactive>:1:19: Couldn't match expected type `Int' against inferred type `[a]' In the second argument of `(<>)', namely `(3 + x)' In the expression: length x <> (3 + x) In the expression: \ x -> length x <> (3 + x) Here, GHC has concluded that x must be [a], and thus 3+x must be [a], which can't be used with <>. -- Dave Menendez <dave@zednenem.com> <http://www.eyrie.org/~zednenem/>