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. If we flip around the expression-order, we get Prelude> \x -> (length x) + (3+x) <interactive>:1:20: 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) ...and yes, technically your explanation of expected/inferred works. But the location/parity depends on the evaluation-order that the type checker uses. The error messages don't seem to specify location precisely enough that I can easily use the information about which type is the 'expected' type and which type is the 'inferred' one. There are usually more chains of inference, let-clauses, etc. through which a contradiction is found, that muddy the issue. hmm. Yes, I think I would appreciate for error messages to tell me exactly which piece of the expression is expected/inferred to be those types. (Is this always a sensible question? or are there sometimes typing-conflicts that can't be / aren't reduced to the expected/inferred type of a particular expression in the code?) -Isaac