#9182: Empty case analysis for function clauses -------------------------------------+------------------------------------ Reporter: dfranke | Owner: Type: feature request | Status: new Priority: low | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Description changed by dfranke: Old description:

Thanks to `-XEmptyCase`, the following is legal:

{{{ module Empty where

data Empty

empty :: Empty -> a empty x = case x of { } }}}

However, if I leave off the last line, GHC will complain that `The type signature for ‘empty’ lacks an accompanying binding`.

I think this program should be accepted. It isn't that I'm not giving a definition for `empty`; it's that I'm defining it to be a function with no accompanying equations, as could be represented in Template Haskell by

$(return [FunD (mkName "empty") []])

Currently, this too is rejected, with GHC complaining `Function binding for ‘empty’ has no equations`. I think this should be legal, with an empty list of clauses in a function definition being treated the same way as an empty list of matches in a case expression.

New description: Thanks to `-XEmptyCase`, the following is legal: {{{ module Empty where data Empty empty :: Empty -> a empty x = case x of { } }}} However, if I leave off the last line, GHC will complain that `The type signature for ‘empty’ lacks an accompanying binding`. I think this program should be accepted. It isn't that I'm not giving a definition for `empty`; it's that I'm defining it to be a function with no accompanying equations, as could be represented in Template Haskell by {{{$(return [FunD (mkName "empty") []])}}} Currently, this too is rejected, with GHC complaining `Function binding for ‘empty’ has no equations`. I think this should be legal, with an empty list of clauses in a function definition being treated the same way as an empty list of matches in a case expression. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9182#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#9182: Empty case analysis for function clauses -------------------------------------+------------------------------------ Reporter: dfranke | Owner: Type: feature request | Status: new Priority: low | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by nomeata): Tricky. I believe the `The type signature for ‘empty’ lacks an accompanying binding.` message is emitted early, possibly during renaming, when there is no way to know that `Empty` is empty. Or are you suggesting to disable this error message completely when `EmptyCase` is active? And what’s wrong with writing `empty x = case x of { }`? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9182#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#9182: Empty case analysis for function clauses -------------------------------------+------------------------------------- Reporter: dfranke | Owner: Type: feature request | Status: new Priority: low | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #11390 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by thomie): * related: => #11390 Comment: A somewhat similar discussion took place in #11390. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9182#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler