Function from loaded module not in scope

Question

I have written two modules. The first one is called DhtTypes:

module DhtTypes (Bencode, encode, TransactionID, Hash20Bytes) where

-- import stuff

class Bencode a where
    encode :: a -> ByteString.ByteString

data TransactionID = TransactionID Int.Int16
data Hash20Bytes = Hash20Bytes [Word.Word8]

-- stuff

The second one is MessageTypes:

module MessageTypes () where

-- import stuff

import DhtTypes

data PingR = PingR TransactionID Hash20Bytes

-- stuff

This is what happen when I load MessageTypes in GHCi:

GHCi, version 7.6.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
[1 of 2] Compiling DhtTypes         ( DhtTypes.hs, interpreted )
[2 of 2] Compiling MessageTypes     ( /home/{path}/MessageTypes.hs, interpreted )
Ok, modules loaded: MessageTypes, DhtTypes.
*MessageTypes> :browse DhtTypes
class Bencode a where
  encode :: a -> ByteString.ByteString
data TransactionID = DhtTypes.TransactionID GHC.Int.Int16
data Hash20Bytes = DhtTypes.Hash20Bytes [GHC.Word.Word8]
*MessageTypes> Hash20Bytes

<interactive>:3:1: Not in scope: data constructor `Hash20Bytes'
*MessageTypes> :l DhtTypes
[1 of 1] Compiling DhtTypes         ( DhtTypes.hs, interpreted )
Ok, modules loaded: DhtTypes.
*DhtTypes> Hash20Bytes [0..10]
Loading package array-0.4.0.1 ... linking ... done.
Loading package deepseq-1.3.0.1 ... linking ... done.
Loading package bytestring-0.10.0.2 ... linking ... done.
Loading package bytestring-builder-0.10.6.0.0 ... linking ... done.
0123456789a
*DhtTypes> 

I have already read ghci not loading function from file and Beginning Haskell - getting “not in scope: data constructor” error, but I still couldn't find an answer.

Answer 1

You're exporting the type Hash20Bytes, but you aren't exporting the constructor Hash20Bytes. You can do so like this

module DhtTypes
    ( Bencode(..)
    , TransactionID(..)
    , Hash20Bytes(..)
    ) where

The (..) exports all constructors/members of a type/typeclass. You can specify a comma separated list of names if you want to only export specific ones, but usually (..) is what's best in my experience.