Can GHCi tell me the type of a local Haskell function?

问题

Is it possible to query the ghci for the type it inferred for a function inside another function?

回答1:

This is a quick and ugly hack, but what I usually do is just use the function in the wrong way and read the error message:

inc x = x + 1
  where
    f (y, z) = y + z
    g = f :: Char

GHCi output:

Couldn't match expected type `Char'
       against inferred type `(t, t) -> t'
In the expression: f :: Char

Although this leaves out the context Num t =>, this usually does provide me with enough information to continue.

回答2:

You might try doing it by setting a breakpoint on it, so the function is in scope from the debugger.

Also I think that EclipseFP can tell you the types of things when you mouse over them, at least some of the time.

回答3:

With GHC 7.8+, just append `asTypeOf` _ to the expression.

See Find out the type of an expression/function with typed holes for a detailed explanation.

来源：https://stackoverflow.com/questions/2128542/can-ghci-tell-me-the-type-of-a-local-haskell-function