Haskell IO monad and do notation -

the following haskell snippit not compile , can't figure out why.

runcompiler :: tc -> io () runcompiler tc = let cp' = cp in               cp'         return ()         cp = compileprog tc 

i getting following error ghci:

    couldn't match expected type `io a0' actual type `string'     in stmt of 'do' block: cp'     in expression:       { cp';            return () }     in expression:       let cp' = cp       in         { cp';              return () } 

any ideas make compile. can't see why not accept () final value given.

when using do notation sequencing 2 statements:

do     action1     action2 

is same action1 >> action2

since >> has type monad m => m -> m b -> m b both action1 , action2 should monadic values.

it appears compileprog function has type tc -> string, while compiler expects tc -> io a a since using in do notation.

you can use let

do      let _ = compileprog tc     return () 

to compile.

if want output returned string, can use putstrln or print:

do     putstrln (compileprog tc)     return () 

since putstrln has type string -> io () can remove return ():

do     putstrln (compileprog tc) 

in fact runcompiler can written as

runcompiler :: tc -> io () runcompiler = putstrln . compileprog