Batch modify files Haskell -

i trying create program adds simple text header every file in directory (and subdirectories). need using built in functions ghc (i can't access cabal).

the type signature of functions are

getrecursivecontents :: filepath -> io [filepath]  addheadertofile :: string -> filepath -> io () 

these both work fine independently struggling use 2 functions in conjunction because of types. assumed using map correct way this, far have been unsuccessful.

addheadertomultiple :: string -> io [filepath] -> io () addheadertomultiple header files = map (addheadertofile header) files 

i understand not work due types used have not found way fix it.

start running io action producing file list:

addheadertomultiple :: string -> io [filepath] -> io () addheadertomultiple header files =    -- files has type io [filepath]    paths <- files    -- paths has type [filepath], can map on    map (addheadertofile header) paths    -- produces [io ()], not io () 

we hit problem in last line, builds list of io actions instead of running them. use helper function:

runinsequence :: [io ()] -> io () runinsequence []     = return ()  -- nothing runinsequence (a:as) = >> runinsequence 

which can simplified to

runinsequence = foldr (>>) (return ()) 

actually, there exists library function doing that: it's called sequence_. our code becomes

import control.monad addheadertomultiple header files =    paths <- files    sequence_ (map (addheadertofile header) paths)    -- produces io (), it's ok 

the combo sequence_ (map ... has own library function well, called mapm_:

addheadertomultiple header files =    paths <- files    mapm_ (addheadertofile header) paths 

this can further refined using >>= directly

addheadertomultiple header files =    files >>= mapm_ (addheadertofile header)