Question

With this code:

main :: FilePath -> FilePath -> IO ()
main wrPath rdPath = do x <- readFile rdPath
                        writeFile wrPath x

I got the following error:

Couldn't match expected type 'IO t0'
            with actual type 'FilePath -> FilePath -> IO()

But the file compiles correctly when I change the name of ‘main’ to something else.

What’s so unique about main and why does its type have to be IO t0?

Answer 1

Because the language spec says so.

A Haskell program is a collection of modules, one of which, by convention, must be called Main and must export the value main. The value of the program is the value of the identifier main in module Main, which must be a computation of type IO t for some type t (see Chapter 7). When the program is executed, the computation main is performed, and its result (of type t) is discarded.