Implement Functor => Applicative => Monad Hierarchy (aka AMP phase 3)

The standard class hierarchy is a consequence of Haskell's historical development, rather than logic. I would therefore like to propose a reform of the Functor, Applicative, and Monad type classes. The new hierarchy is logical, eliminates many duplicate names from the standard type class definitions, and removes the need for boilerplate Monad -> Applicative instance declarations.

The proposal is detailed in the wiki, along with an example of a legacy module to provide some backwards-compatibility.

Trac metadata

Trac field	Value
Version	7.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import core libraries labels

Trac metadata

Trac field	Value
CC	→ gideon@accursoft.com

Formally proposing this on the mailing lists, with a discussion deadline of 1 February.

Attached file ghc_new_monad_hierarchy.dpatch (download).

Prepare GHC for the new monad hierarchy

Attached file base_new_monad_hierarchy.dpatch (download).

Patch for base that implements the new monad hierarchy

Attached file functor_and_applicative_instance_HappyIdentity.dpatch (download).

Patch for happy that generates a Functor and Applicative instance for HappyIdentity

All my patches can also be pulled from my publicly available ghc repository:

darcs pull http://bifunctor.homelinux.net/~bas/ghc/
darcs pull http://bifunctor.homelinux.net/~bas/ghc/libraries/base
darcs pull http://bifunctor.homelinux.net/~bas/ghc/...

Also see the generated haddock documentation of the base library with the patches applied.

Please note that the attached patches only implement the new Applicative => Monad hierarchy, but do not change any names (as proposed on the wiki page). The deprecation/renaming of redundant definitions may be proposed in a separate ticket.

changed milestone to %Not GHC

mentioned in issue #4881 (closed)

closed

Proposal tickets are no longer needed as part of the library submissions process. Instead, a normal ticket should be created once consensus has been achieved. Please see the process description for details.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedInvalid

mentioned in issue #8003 (closed)

mentioned in issue #8004 (closed)

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Trac field	Value
CC	gideon@accursoft.com →

reopened

changed milestone to %7.10.1

changed weight to 7

added Ttask label and removed Tbug label

Reusing this old AMP ticket so I don't have to create a new one to track AMP progress;

AMP phase 1 was covered by #8004 (closed), while this ticket is about phase 3 (actually implementing the change)

The AMP was implemented in a single monolithic commit, followed by AMP-related follow-up patches fixing up things as well as generalising some type signatures from Monad to Applicative:

• d94de872 ("Make Applicative a superclass of Monad")
• 65f887e1 ("base: Add some notes about the default impl of (>>)")
• 6477b3dc ("testsuite: AMPify ioprof.hs")
• 29e50da4 ("testsuite: AMPify T3001-2")
• f636faa7 ("Set default-impl of mapM/sequence methods to traverse/sequenceA")
• a07ce165 ("Generalise Control.Monad.{when,unless,guard}")
• bf332910 ("Generalise guard for real this time")
• a1b53914 ("testsuite: fix T5751 build failure (AMP)")
• b30b185e ("testsuite: fix T1735_Help/State.hs build failure (AMP)")
• abba3812 ("Improve Applicative definitions")

Trac metadata

Trac field	Value
Type	Bug → Task
Priority	normal → high
Resolution	ResolvedInvalid → Unresolved
Differential revisions	- → D13
Related	→ #8004 (closed)

changed title from New Functor => Applicative => Monad Hierarchy to Implement Functor => Applicative => Monad Hierarchy (aka AMP phase 3)

mentioned in commit a741e69a

Replying to [#4834 (closed) Herbert Valerio Riedel <hvr@…>]:

In [changeset:"a741e69a/ghc"]:

1. ..also merged to ghc-7.10 via 262f7a85

closed

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

mentioned in issue #10071

added Tbug label

added report impact label

added Phigh label

mentioned in merge request !6876 (closed)