Question

I am trying to parse a simple binary file in Haskell with the Data.Binary.Get monad.

A simplified version of my code looks like this:

data MsgString = Definition_msg {
      msg_no        :: Word16
    } deriving (Show)

parseDef :: Get MsgString
parseDef = do
    msg_no   <- getWord16le
    return $ Definition_msg msg_no

parseMain :: Get [MsgString]
parseMain =  do
      bit <- getWord8
      msg <- parseDef
      return msg:parseMain

And the error I get is the following:

Prelude> :l example.hs 
[1 of 1] Compiling Main             ( example.hs, interpreted )

example.hs:23:17:
    Couldn't match expected type `[m MsgString]'
           against inferred type `Get [MsgString]'
    In the second argument of `(:)', namely `parseMain'
    In the expression: return msg : parseMain
    In the expression:
        do { bit <- getWord8;
             msg <- parseDef;
               return msg : parseMain }
Failed, modules loaded: none.

Can anyone see what I do wrong?

Thanks!

Answer 1

The issue is your last line, which parses as:

(return msg) : parseMain

But that really isn’t the only problem. parseMain is of type Get [MsgString] when you really just want a [MsgString] so you must run the monadic action first:

parseMain :: Get [MsgString]
parseMain =  do
    bit <- getWord8
    msg <- parseDef
    rest <- parseMain
    return (msg : rest)

Notice this will get an infinite list of MsgString‘s and won’t terminate without an exception. Perhaps you intended to have an if statement guarding that recursive call to parseMain?