#8753: Import constructor but not the data type -------------------------------------+------------------------------------ Reporter: andreas.abel | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Old description:
How do I import a constructor but not its data type? The constructor might have the name of a data type which I do not want to bring into scope.
Use case: Library (A) defines a parametrized data type (D a) which I want to use in my code (B) in instantiated form (D Int) but with the same name (D). I want to pattern match against inhabitants of type D, so I need the constructor(s) in scope.
Here is how it could look like if import lists let me specify whether I want to import a type or a constructor with name "D".
module A where
data D a = D a
module B where
import A (constructor D) import qualified A D = A.D Int
f :: D -> Int f (D x) = x
Haskell has different name spaces for type and constructors, but does not let the user talk about these name spaces where it matters, namely in import/export lists. At least I found no documentation instructing me how to do this.
New description: How do I import a constructor but not its data type? The constructor might have the name of a data type which I do not want to bring into scope. Use case: Library (`A`) defines a parametrized data type (`D a`) which I want to use in my code (`B`) in instantiated form (`D Int`) but with the same name (`D`). I want to pattern match against inhabitants of type `D`, so I need the constructor(s) in scope. Here is how it could look like if import lists let me specify whether I want to import a type or a constructor with name "`D`". {{{#!haskell module A where data D a = D a }}} {{{#!haskell module B where import A (constructor D) import qualified A D = A.D Int f :: D -> Int f (D x) = x }}} Haskell has different name spaces for type and constructors, but does not let the user talk about these name spaces where it matters, namely in import/export lists. At least I found no documentation instructing me how to do this. -- Comment (by hvr): (fixed-up wiki markup in ticket description) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8753#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler