[Haskell-beginners] More Deserialization Woes

[Tom Hobbs]

Hi guys,

Thanks for all the previous help I'm having with serialization in Haskell, I
couldn't have gotten as far as I have without the help from this group. Many
thanks!

Now I've put you in a good mood, maybe you can help me with my latest
problem... :-)

I have the following code which reads strings from a handle;

import qualified Data.ByteString.Lazy.UTF8 as UTF

readNames 0 _ = []
readNames n h = do
                           length <- fmap (fromIntegral . runGet
getWord32be) $ L.hGet h 4
                           name <- L.hGet h length
                           (UTF.toString name) : readNames (n-1) h

Where n is the number of names remaining to read and h is the handle to the
stream that I'm reading from. It is my intention that this function would
return a [String] of all the names. I'm sure further explainations probably
aren't necessary, but just in case;

- "length" becomes equal to some number (represented by four bytes) which
describes the length of the string to read
- "name" becomes the string I'm trying to read
- Then I add that String to the list and go again

But the problem I'm getting is this;

Couldn't match expected type `[a]' against inferred type `IO b'
In a stmt of a 'do' expression:
                           length <- fmap (fromIntegral . runGet
getWord32be) $ L.hGet h 4
In the expression:
                           do { length <- fmap (fromIntegral . runGet
getWord32be)
                           $ L.hGet h 4;
                           name <- L.hGet h length;
                           (UTF.toString name) : readNames (n - 1) h }

In the definition of `readNames':
                           readNames n h
                           = do { length <- fmap (fromIntegral . runGet
getWord32be)
                           $ L.hGet h 4;
                           name <- L.hGet h length;
                           (UTF.toString name) : readNames (n - 1) h }

I (think I) understand what the message is saying, but not why it's
occurring. I read the documentation as "UTF8.tostring" essentially rips the
[Char] out of the ByteString and returns that, so where does the "IO b" come
into it? Is it complaining because the reads from the handle are IO
operations and it's expecting me to use the IO monad somewhere? What i want
to be able to do is do the IO stuff inside the monad, but end up with a
[String] which I can then use in a pure function.

RWH and Learn You A Good Haskell all cover some stream reading, but their
examples are to different from what I'm trying to do that I'm getting stuck.

Any pointers are very gratefully accepted.

Many thanks,

Tom
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