#4879: Deprecate exports
---------------------------------+------------------------------------------
Reporter: basvandijk | Owner:
Type: feature request | Status: new
Priority: normal | Component: Compiler
Version: 7.0.1 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
---------------------------------+------------------------------------------
== Motivation ==
During the library submission process there's sometimes the desire to have
the ability to deprecate an export from a module.
For example
[http://article.gmane.org/gmane.comp.lang.haskell.libraries/14524 during
the discussing] about ticket #4422, I would have liked the ability to
deprecate the exports of the `String` functions: `lines`, `words`,
`unlines` and `unwords` from `Data.List` in favour of importing them from
`Data.String`. However I wasn't able to do so, so these exports remain.
Similarly,
[http://article.gmane.org/gmane.comp.lang.haskell.libraries/14925 during
the discussion] about ticket #4865, Ian also desired to deprecate the
export of `catch` from `System.IO.Error` but was unable to do so.
== Syntax ==
To deprecate an export simply place a `DEPRECATE` pragma for the export
inside the export list, as in:
{{{
module Data.List
( ...
{-# DEPRECATE lines "Exported from Data.String instead" #-}
, lines
...
) where
...
}}}
Another design might be to have a different pragma as in:
{{{
{-# DEPRECATE_EXPORT lines "Exported from Data.String instead" #-}
}}}
But I find the former much prettier and more obvious.
== Semantics ==
If the `lines` export from `Data.List` is deprecated the following should
raise deprecation warnings:
* Directly importing a deprecated export:
{{{
import Data.List (lines)
}}}
* Referring to a deprecated export:
{{{
import Data.List
foo = lines
}}}
If you import the same symbol from different modules and only some of them
are deprecated exports then referring to the symbol won't give a
deprecation warning. For example the following should not give deprecation
warnings:
{{{
import Data.List
import Data.String
foo = lines
}}}
=== What exports can be deprecated? ===
* Functions.
* Types.
* Classes.
* Constructors. Possible syntax:
{{{
module A
( {-# DEPRECATE T(C1) "The export of C1 is deprecated" #-}
T(C1, C2, C3)
) where
}}}
* Modules. Possible syntax:
{{{
module A
( {-# DEPRECATE module B "The export of module B is deprecated" #-}
module B
) where
}}}
The semantics of deprecating a module export is that you get deprecation
warnings for all symbols from module `B` that you refer to inside a module
that imports `A`. (Does that make sense?)
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4879>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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