Neater binary file processing in Haskell

Question

This is a general question inspired by a particular piece of code I wrote that I'm not happy with. I'm using Data.Binary.Get to grab some data from a binary file. Some of the code looks a bit like this

import Data.Binary.Get

data Thing = Thing
  {
    aaa :: Integer,
    bbb :: Integer,
    ccc :: Integer
  } deriving (Show)

getThing :: Get Thing
getThing = do
  laaa <- getWord8 >>= \x -> return (toInteger x)
  lbbb <- getWord16host >>= \x -> return (toInteger x)
  lccc <- getWord32host >>= \x -> return (toInteger x)
  return $ Thing laaa lbbb lccc

The "getThing" function is really long. I am hoping there is a nice way to do something like the following pseudocode or maybe something even more concise.

[laaa, lbbb, lccc] <- MAGIC [getword8, getword16, getWord32] >>= \x -> return (toInteger x)

What have you got?

Bergi · Accepted Answer · 2019-11-17 18:57:34Z

4

I would write

getThing :: Get Thing
getThing = Thing <$> intFrom getWord8 <*> intFrom getWord16 <*> intFrom getWord32
  where
    where intFrom x = toInteger <$> x

The magic you are looking for is known as sequence, but you can't put IO Word8, IO Word16 and IO Word32 in the same list:

getThing :: Get Thing
getThing = do
  [laaa, lbbb, lccc] <- sequence [toInteger <$> getword8, toInteger <$> getword16, toInteger <$> getWord32]
  return $ Thing laaa lbbb lccc

edited Nov 17, 2019 at 18:57

answered Nov 17, 2019 at 16:52

Bergi

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4 Comments

K. A. Buhr Over a year ago

Also, note that for the version using the shorthand intFrom, for obscure reasons you'll need to either turn on the {-# LANGUAGE NoMonomorphismRestriction #-} pragma or give intFrom an explicit type signature (e.g., intFrom :: (Functor f, Integral a) => f a -> f Integer).

CodeWash Over a year ago

Thanks! I went with your first suggestion. It's much easier to read than what I had when laid out over multiple lines with comments on each line.

Bergi Over a year ago

@K.A.Buhr Thanks, I completely forgot about that. Not putting intFrom in a top-level declaration should work as well, right?

K. A. Buhr Over a year ago

@Bergi, that would have the same problem, as I think you discovered. But an even nicer solution is probably to write where intFrom x = toInteger <$> x. Giving intFrom an explicit argument changes the typing rules slightly, so the monomorphism restriction no longer applies.

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