#12556: `stimes` adds extra power to Semigroup -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: task | Status: new Priority: normal | Milestone: Component: libraries/base | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect result | Unknown/Multiple at runtime | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by chris-martin): I see three options: 1. Add a `Semigroup` law "`stimes 0 x` = ⊥". This would require introducing some strictness on the first argument and changing some (many?) instances. 2. Add a `Semigroup` law "If `stimes 0 x` = `i` ≠ ⊥, then `i` must be an identity". I'd guess all of the instances already adhere to this. 3. Add a `Semigroup` "un-law" that simply warns "For `n` less than 1, `stimes n x` is not defined and may produce anything." This certainly requires no code changes. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12556#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler