Import item suggests constructors/methods [GHC-99623]

Flag: -Wdodgy-imports
Enabled by: -Wextra

When importing identifiers from a module, all in-scope constructors (for a type) or the in-scope methods (for a typeclass) can also be imported with the T(..) syntax. However, if T was exported abstractly as only T - either its constructors/methods are not exported, or it lacks any - then this import item suggests that T has in-scope constructors/methods when it has none.

Examples

Import item suggests constructors/methods

When importing TypeWithoutVisibleCtrs from module HiddenConstructors, the import list item in module DodgyImports suggests that this type has associated in-scope constructor(s). However it does not (the constructors are out of scope). The same is true of TypeWithNoCtrs; the import item suggests it has constructors whereas it has none (either in or out of scope).

The solution is to import the type abstractly (without (..)) to match its exporting.

Error Message

DodgyImports.hs:3:1: warning: [-Wdodgy-imports] [GHC-99623]
    The import item ‘TypeWithoutVisibleCtrs(..)’ suggests that
    ‘TypeWithoutVisibleCtrs’ has (in-scope) constructors or class methods,
    but it has none
  |
3 | import HiddenConstructors ( TypeWithoutVisibleCtrs(..), TypeWithNoCtrs(..) )
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

DodgyImports.hs:3:1: warning: [-Wdodgy-imports] [GHC-99623]
    The import item ‘TypeWithNoCtrs(..)’ suggests that
    ‘TypeWithNoCtrs’ has (in-scope) constructors or class methods,
    but it has none
  |
3 | import HiddenConstructors ( TypeWithoutVisibleCtrs(..), TypeWithNoCtrs(..) )

DodgyImports.hs

Before

module DodgyImports where

import HiddenConstructors ( TypeWithoutVisibleCtrs(..), TypeWithNoCtrs(..) )

After

module DodgyImports where

import HiddenConstructors ( TypeWithoutVisibleCtrs, TypeWithNoCtrs )

HiddenConstructors.hs

Before

module HiddenConstructors ( TypeWithoutVisibleCtrs, TypeWithNoCtrs ) where

data TypeWithoutVisibleCtrs = Ctr1 Int | Ctr2 | Ctr3 Bool Bool

data TypeWithNoCtrs

After

module HiddenConstructors ( TypeWithoutVisibleCtrs, TypeWithNoCtrs ) where

data TypeWithoutVisibleCtrs = Ctr1 Int | Ctr2 | Ctr3 Bool Bool

data TypeWithNoCtrs