#14252: ApplicativeDo: Add compiler message about irrefutable pattern matches and Monad constraints -------------------------------------+------------------------------------- Reporter: mutantmell | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by bgamari: Old description:

Add an error message to the compiler when ApplicativeDo desugaring: * requires a Monad constraint when a Functor/Applicative constraint is expected * Adding a lazy pattern match could allow the Functor/Applicative constraint

bgamari suggests something like the following message:

{{{#!hs ApplicativeDo is enabled yet Monad is required due to irrefutable pattern match; perhaps you want to make the match refutable by adding a ~? }}}

Background:

In GHC 8.2.1, ApplicativeDo desugaring was changed to require monad constraints when using a strict pattern match (see #13875 for details). While this behavior is a requirement for maintaining the intended semantics of ApplicativeDo, it is both a breaking change, and somewhat unintuitive (see #14249). Adding a message would provide both clarification around the requirement, and provide a simple resolution for the common use case.

New description: Add an error message to the compiler when ApplicativeDo desugaring: * requires a Monad constraint when a Functor/Applicative constraint is expected * Adding a lazy pattern match could allow the Functor/Applicative constraint bgamari suggests something like the following message: {{{ ApplicativeDo is enabled yet Monad is required due to irrefutable pattern match; perhaps you want to make the match refutable by adding a ~? }}} Background: In GHC 8.2.1, ApplicativeDo desugaring was changed to require monad constraints when using a strict pattern match (see #13875 for details). While this behavior is a requirement for maintaining the intended semantics of ApplicativeDo, it is both a breaking change, and somewhat unintuitive (see #14249). Adding a message would provide both clarification around the requirement, and provide a simple resolution for the common use case. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14252#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#14252: ApplicativeDo: Add compiler message about irrefutable pattern matches and Monad constraints -------------------------------------+------------------------------------- Reporter: mutantmell | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: 8.4.1 Component: Compiler | Version: 8.2.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): When you add warnings like this you end up needing another flag to suppress the warning. What if the user says "No, I didn't to make the match irrefutable; quit bugging me". And then a per-module suppression flag is a bit of a blunt instrument. I'm not sure what's best; it would be good to get feedback from `ApplicativeDo` users. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14252#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#14252: ApplicativeDo: Add compiler message about irrefutable pattern matches and Monad constraints -------------------------------------+------------------------------------- Reporter: mutantmell | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: 8.4.1 Component: Compiler | Version: 8.2.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by mutantmell): I changed the wording of the ticket to be more clear with my original intent: add a message when a user provided signature clashes with what ApplicativeDo allows, similar to when enabling an extension would allow code to compile. This should be less obtrusive for users who don't want to be bugged, while providing a message that would help people make their code compile. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14252#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#14252: ApplicativeDo: Add compiler message about irrefutable pattern matches and Monad constraints -------------------------------------+------------------------------------- Reporter: mutantmell | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: 8.8.1 Component: Compiler | Version: 8.2.1 Resolution: | Keywords: ApplicativeDo Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by watashi): * cc: watashi (added) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14252#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler