Question

Simple question. Is it possible to check the type of a variable that is only alive within a function?

For example:

main = do
       x <- something

How can I check the type of x?

I can’t do :type x in ghci because x is not global.

Answer 1

Another way (quite similar to RobAgar’s and hacky as well) is to explicitly specify some wrong type for the local variable in question, e.g.:

main = do
  x <- getArgs
  x :: () -- let's cast it to unit! ;)
  print $ head x

Then ghci will give us an error:

Couldn't match expected type `()' with actual type `[String]'

which shows that the actual type of “x” is [String].