Haskell IO monad和表示法

Question

The following Haskell snippit will not compile and I can't figure out why.

runCompiler :: TC -> IO ()
runCompiler tc = let cp' = cp in 
    do 
        cp'
        return ()
    where
    cp = compileProg tc

I am getting the following error from GHCi:

    Couldn't match expected type `IO a0' with actual type `String'
    In a stmt of a 'do' block: cp'
    In the expression:
      do { cp';
           return () }
    In the expression:
      let cp' = cp
      in
        do { cp';
             return () }

Any ideas what to do to make it compile. I can't see why it will not accept () as the final value given.

Answer 1

When using do notation sequencing two statements:

do
    action1
    action2

is the same as action1 >> action2

since >> has type Monad m => ma -> mb -> mb both action1 and action2 should be monadic values.

It appears your compileProg function has type TC -> String , while the compiler expects it to be TC -> IO a for some a since you are using it in do notation.

You can use a let

do 
    let _ = compileProg tc
    return ()

to get it to compile.

If you want to output the returned string, you can use putStrLn or print :

do
    putStrLn (compileProg tc)
    return ()

since putStrLn has type String -> IO () you can remove the return () :

do
    putStrLn (compileProg tc)

In fact runCompiler can be written simply as

runCompiler :: TC -> IO ()
runCompiler = putStrLn . compileProg