#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

#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: -------------------------------------+------------------------------------ Changes (by simonpj): * cc: cactus (added) Comment: You are right that there is no good way to do this, and that the lack is unfortunate. It is, however, possible in GHC 7.8. You can say {{{ import A( pattern D ) }}} provided you use `LANGUAGE PatternSynonyms`. Here `D` is a data constructor, but that's just a degenerate pattern synonym. Similarly in hiding and export lists. Does that help? I see that this point does not (yet) show up in the user manual, and it should. Gergo, might you fix that? Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8753#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#8753: Import constructor but not the data type -------------------------------------+------------------------------------ Reporter: andreas.abel | Owner: Type: feature request | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Changes (by nomeata): * status: new => closed * resolution: => fixed -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8753#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#8753: Import constructor but not the data type -------------------------------------+------------------------------------ Reporter: andreas.abel | Owner: Type: task | Status: new Priority: normal | Milestone: Component: Documentation | 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: -------------------------------------+------------------------------------ Changes (by nomeata): * status: closed => new * resolution: fixed => * component: Compiler => Documentation * type: feature request => task Comment: Doesn’t seem like it. http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html #pattern-synonyms mentions `pattern Foo` in export/import list, but does not say that proper constructors can also be addressed this way. I guess it should also be mentioned in http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html #explicit-namespaces Anywhere else where this should be mentioned? (Reopening as a task.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8753#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#8753: Import constructor but not the data type -------------------------------------+------------------------------------ Reporter: andreas.abel | Owner: Type: task | Status: new Priority: normal | Milestone: Component: Documentation | 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: -------------------------------------+------------------------------------ Comment (by Simon Peyton Jones ): In [changeset:"59cdb992df5bd8bdc563b30c2103c323a7d57f15/ghc"]: {{{ #!CommitTicketReference repository="ghc" revision="59cdb992df5bd8bdc563b30c2103c323a7d57f15" Document explicit import/export of data constructors (Trac #8753) I also added sub-sections to the pattern synonym documentation }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8753#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler