Cannot import type constructor from installed package in Haskell

Question

I'm trying to use the Stream type defined in this package. After installing with with cabal, I tried defining the tail function on streams as a quick test :

import Stream  

tail' :: Stream a -> Stream a
tail' (Cons x s) = s

which gives me this error message :

test.hs:3:14: error:
    Not in scope: type constructor or class `Stream'
    Perhaps you meant `StreamT' (imported from Stream)

test.hs:3:26: error:
    Not in scope: type constructor or class `Stream'
    Perhaps you meant `StreamT' (imported from Stream)

test.hs:4:12: error: Not in scope: data constructor `Cons'
Failed, modules loaded: none.

after some searches, I thought the problem might be that importing the module does not automatically import the type constructor Stream and constructor Cons with it. So I changed it to

import Stream (Stream, Cons)

tail' :: Stream a -> Stream a
tail' (Cons x s) = s

after which I get

test.hs:1:20: error: Module `Stream' does not export `Stream'

test.hs:1:28: error: Module `Stream' does not export `Cons'
Failed, modules loaded: none.

and this is puzzling. Do I have to alter the installed package and add Stream and Cons to its export list? Or am I not importing the module correctly?

The package you link only has Data.Stream. You must be importing something else. — András Kovács

Daniel Wagner Daniel Wagner · Accepted Answer · 2016-08-29T20:43:00

You are importing the wrong module; looking at the top of your link, the module name is Data.Stream. (Stream is the name of the package -- it names a collection of modules to be installed.) So things should work better if you write

import Data.Stream

tail' :: Stream a -> Stream a
tail' (Cons x s) = s

The fact that the incantation import Stream worked at all indicates to me that you have another package installed that provides this module, though I'm not sure which.

Cannot import type constructor from installed package in Haskell

1 Answers