Question

Simple question.

This compiles:

module Example where

import Network

port :: PortID
port = PortNumber 3001

And this does not:

module Example where

import Network (PortID, PortNumber)

port :: PortID
port = PortNumber 3001

GHC says:

Example.hs:6:8: Not in scope: data constructor `PortNumber'

Why?

Answer 1

It has to be

import Network (PortID(PortNumber))

as PortNumber seems to be a constructor of PortID. The other import simply imports all of Network and hence PortNumber is found.