#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: | -------------------------------------+------------------------------------- Description changed by mutantmell: 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:
{{{ 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: * A user-provided signature specifies a Functor/Applicative constraint * ApplicativeDo desugaring requires a Monad constraint * 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:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler