[Haskell-beginners] Expected vs. Inferred Type

[Arlen Christian Mart Cuss]

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.