#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:
(see description in Phab:D1516 for the meantime)
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 = ... (>>=) = ... (>>) = ... }}} 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. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11128#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler