On Sun, 2011-06-05 at 17:44 -0800, Christopher Howard wrote:
Could someone explain the difference between an expected type and an inferred type as mentioned in a compiler error message? I've never heard definitions proper, and so I tend to get confused when the compiler says there is a difference between the type it expected and the type it inferred.
Hi Christopher, The expected type is the type expected by the compiler in that position; e.g. the argument of `not' should be a Bool:
:t not not :: Bool -> Bool
Hence in the expression `not x', `x' is *expected* to be a Bool. The inferred (or actual) type is the type that Haskell has determined the value *actually* is. For instance:
not "abc"
<interactive>:1:5: Couldn't match expected type `Bool' with actual type `[Char]' In the first argument of `not', namely `"abc"' In the expression: not "abc" In an equation for `it': it = not "abc"
The actual type of the argument to `not' is [Char], but the compiler needs `Bool'. tl;dr: "expected" is what the compiler wants, "inferred" is what you gave it.