#11128: New `-fwarn-noncanonical-monad-instances` warning -------------------------------------+------------------------------------- Reporter: hvr | Owner: hvr Type: feature request | Status: patch Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.10.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D1516 Wiki Page: | -------------------------------------+------------------------------------- Comment (by nomeata): Do you intentionally disallow `return x = pure x`, which some people might write? (No strong opinion here, having precisely one official way is probably desirable, but let’s at least document that it has been given thought.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11128#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#11128: New `-fwarn-noncanonical-monad-instances` warning -------------------------------------+------------------------------------- Reporter: hvr | Owner: hvr Type: feature request | Status: patch Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.10.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D1516 Wiki Page: | -------------------------------------+------------------------------------- Comment (by Herbert Valerio Riedel ): In [changeset:"f09f2470a76bb08b7f51d2f5663daa672b86f618/ghc" f09f2470/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="f09f2470a76bb08b7f51d2f5663daa672b86f618" Implement new `-fwarn-noncanonical-monad-instances` Warn about incoherent/non-canonical 'Applicative'/'Monad' instance declarations. Specifically the following invariants are checked: In 'Monad' instances declarations warn if the any of the following conditions does not hold: * If `return` is overridden it must be canonical (i.e. `return = pure`). * If `(>>)` is overridden it must be canonical (i.e. `(>>) = (*>)`). In 'Applicative' instance declarations: * Warn if 'pure' is defined backwards (i.e. `pure = return`). * Warn if '(*>)' is defined backwards (i.e. `(*>) = (>>)`). NB, this warning flag is not enabled via `-Wall` nor `-Wcompat`. This addresses #11128 Reviewers: quchen, austin, bgamari Reviewed By: bgamari Differential Revision: https://phabricator.haskell.org/D1516 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11128#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#11128: New `-fwarn-noncanonical-monad-instances` warning -------------------------------------+------------------------------------- Reporter: hvr | Owner: hvr Type: feature request | Status: patch Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.10.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D1516 Wiki Page: | -------------------------------------+------------------------------------- Description changed by hvr: Old description:

When declaring `Applicative` and `Monad` instances, there's a degree of freedom in which way to define `return`,`pure`,`(>>)`,`(*>)`. For instance, defining

{{{#!hs instance Applicative T1 where pure = return (<*>) = ap

instance Monad T1 where return = ... (>>=) = ... (>>) = ... }}}

is ok, but it's leaves `(*>)` with a possibly less optimised version than `(>>)`. This can cause performance regressions when generalising code from `Monad` to `Applicative`.

Moreover, starting with `base-4.8`, the `return` method gained a default implementation `return = pure` which follows the preferred or "canonical" direction of having implementations flow from superclasses to their subclasses.

So this warning is a "lint"-style check to help detect `Monad` instances where the definitions of `return`/`(>>)` are not canonical, i.e. don't match `return = pure` and `(>>) = (*>)` respectively.

New description: When declaring `Applicative` and `Monad` instances, there's a degree of freedom in which way to define `return`,`pure`,`(>>)`,`(*>)`. For instance, defining {{{#!hs instance Applicative T1 where pure = return (<*>) = ap instance Monad T1 where return = ... (>>=) = ... (>>) = {- optimised impl -} }}} is ok, but this leaves `(*>)` with a possibly less optimised version than `(>>)`. This can cause performance regressions when generalising code from `Monad` to `Applicative`. Moreover, starting with `base-4.8`, the `return` method gained a default implementation `return = pure` which follows the preferred or "canonical" direction of having implementations flow from superclasses to their subclasses. A proper "canonical" definition of `T1` is consequently: {{{#!hs instance Applicative T1 where pure = ... (<*>) = ap (*>) = {- optimised impl -} instance Monad T1 where return = pure -- can be left off since base-4.8 (>>=) = ... (>>) = (*>) -- NB: default impl of (>>) /= (*>) }}} So this warning is a "lint"-style check to help detect `Monad` instances where the definitions of `return`/`(>>)` are not canonical, i.e. don't match `return = pure` and `(>>) = (*>)` respectively. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11128#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#11128: New `-fwarn-noncanonical-monad-instances` warning -------------------------------------+------------------------------------- Reporter: hvr | Owner: hvr Type: feature request | Status: closed Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.10.2 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D1516 Wiki Page: | -------------------------------------+------------------------------------- Comment (by Herbert Valerio Riedel ): In [changeset:"986ceb1679b501414b996c520b08ce929a40f94c/ghc" 986ceb16/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="986ceb1679b501414b996c520b08ce929a40f94c" Implement new `-fwarn-noncanonical-monoid-instances` This is similiar to the `-fwarn-noncanonical-monad-instances` warning implemented via #11128, but applies to `Semigroup`/`Monoid` instead and the `(<>)`/`mappend` methods (of which `mappend` is planned to move out of `Monoid` at some point in the future being redundant and thus error-prone). This warning is contained in `-Wcompat` but not in `-Wall`. This addresses #11150 Reviewed By: quchen Differential Revision: https://phabricator.haskell.org/D1553 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11128#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler