Re: [Haskell-beginners] How to get IO String from Network.Socket.ByteString.recv method

A bit late to the party, but to build on the previous answer, you're going to run into the same problem with the Text data type, which has both a lazy variant and a strict (?) variant. The functions to convert from/to the strict/lazy variant are usually in the lazy module: *Data.Text.Lazy.toStrict* and *Data.Text.Lazy.fromStrict*. *Data.ByteString.Lazy.toStrict* and *Data.ByteString.Lazy.toStrict*. It's a common use-case to want to save the Text / ByteString to a file, and in those cases you can find helper functions such as *Data.ByteString.Lazy.writeFile*. On Sun, May 20, 2018 at 6:56 PM, Daniel Bergey wrote:

Network.Socket.ByteString.recv uses the strict ByteString from Data.ByteString, not the lazy one from Data.ByteString.Lazy. So you want the `unpack` from Data.ByteString.Char8, rather than Data.ByteString.Lazy.Char8.

I never remember which functions return strict or lazy ByteString. I find the easiest way to check is to open the online docs and see where the `ByteString` link points:

https://hackage.haskell.org/package/network-2.7.0.0/docs/ Network-Socket-ByteString.html#v:recv

points to:

https://hackage.haskell.org/package/bytestring-0.10.8.2/ docs/Data-ByteString.html#t:ByteString

hope this helps, bergey

On 2018-05-20 at 07:52, Dinesh Amerasekara wrote:

...

Hi,

I am unable to compile the below code.

import Network.Socket hiding(recv) import Network.Socket.ByteString as S (recv) import qualified Data.ByteString.Lazy.Char8 as Char8

getMessage :: Socket -> IO String getMessage sock = Char8.unpack <$> S.recv sock 8888

It gives the below error.

Couldn't match type ‘Data.ByteString.Internal.ByteString’ with ‘ByteString’ NB: ‘ByteString’ is defined in ‘Data.ByteString.Lazy.Internal’ ‘Data.ByteString.Internal.ByteString’ is defined in ‘Data.ByteString.Internal’ Expected type: IO ByteString Actual type: IO Data.ByteString.Internal.ByteString

In the second argument of ‘(<$>)’, namely ‘recv sock 8888’ In the expression: unpack <$> recv sock 8888 In an equation for ‘getMsg’: getMsg sock = unpack <$> recv sock 8888

Can somebody tell me how I can return the IO String using Network.Socket.ByteString.recv?

Best Regards, Dinesh.

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