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# Interface deprecation mechanisms

This page serves as a place to collect proposals, tickets, open problems, and ideas regarding mechanisms for deprecating program interfaces.

For motivation for these mechanisms one can look to a number of recently-considered proposals for reworking the core libraries, for instance the [Applicative-Monad Proposal](https://wiki.haskell.org/Functor-Applicative-Monad_Proposal#Future-proofing_current_code).

## Warning on import of `DEPRECATED` definitions

GHC has for a long time allowed users to attach `DEPRECATED` pragmas to definitions, defining a warning message which will be shown to clients when a use-site of the marked definition is encountered.

Sadly, this design doesn't account for instances where a deprecated definition is explicitly imported but not actually *used*. #2119 argues that GHC should issue a warning when an explicit import of a deprecated definition is found.

**SimonM** If you import a `DEPRECATED` identifier, then there are two cases: either it is mentioned somewhere, in which case the deprecation warning will be shown, or it is not mentioned, in which case there will be an unused import warning (provided it is enabled), so this seems less useful than the other suggestions on this page. **End SimonM**

TODO Write formal specification.

## Deprecating exports

GHC's existing `DEPRECATED` pragma only covers definitions, not their visibility. #4879 argues that GHC should allow `DEPRECATED` pragmas to be attached to names in export lists, meaning that the definition itself isn't deprecated (allowing warning-free uses within the defining module), only its role as an outward-facing interface.

TODO Write specification

## Class method deprecation

As of GHC 7.10 there is no effective way to demote a typeclass method to a normal top-level binding (see #10071). This would be useful in cases such as

- removing the `return` method from `Monad` as proposed by the [Monad of No Return proposal](https://mail.haskell.org/pipermail/libraries/2015-September/026121.html)
- removing the `mappend` method from `Monoid` as proposed in [Proposal/SemigroupMonoid](proposal/semigroup-monoid)

A concrete proposal for allowing `DEPRECATED` pragmas to be attached to class methods can be found at [Design/DeprecationMechanisms/TypeClassMethods](design/deprecation-mechanisms/type-class-methods).

## `REMOVED` Pragma

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