[GHC] #9115: The kind of (=>)
#9115: The kind of (=>) ------------------------------------+------------------------------------- Reporter: kosmikus | Owner: Type: feature request | Status: new Priority: low | Milestone: Component: Compiler | Version: 7.8.2 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- This is mainly a request for clarification, and not very important. Is there any good (theoretical, implementation, practical, ...) reason why (with `ConstraintKinds`) enabled, the `=>` symbol on the type-level is still treated as a built-in syntactic construct rather than a type-level operator with {{{ GHCi> :kind (=>) (=>) :: Constraint -> * -> * }}} Note that I'm not actually proposing to make it re-definable, but to treat it similarly to the `(->)` type operator. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9115 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#9115: The kind of (=>) -------------------------------------+------------------------------------ Reporter: kosmikus | 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 goldfire): Here's a reason: normally, if we know `(a b c) ~ (d e f)`, it is reasonable to conclude `a ~ d`, `b ~ e` and `c ~ f`. But, what if `a` is really `(=>)`? Could `b` become `(Eq a, Eq [a])` and `e` become `Eq [a]`? Is it the case that `(Eq a, Eq [a]) ~ Eq [a]`? These are logically equivalent, but are they equal in the sense of `(~)`? I would hope not. Indeed, even asking whether `(a => b) ~ (c => d)` right now is forbidden. There may be a reasonable way forward here, but it's not abundantly obvious. If you want More Thought put into this, do you have a use case in mind? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9115#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#9115: The kind of (=>) -------------------------------------+------------------------------------ Reporter: kosmikus | 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 kosmikus): Thanks a lot for the answer. This is indeed the kind of problem I was suspecting behind the limitation, but I wasn't quite sure what issues there are. I don't need this in any way. I've just been explaining constraint kinds in the context of Haskell courses and talks a couple of times recently, and I always thought it would be nice to be able to get GHCi to output the kind of `=>`. But I completely understand that it may be problematic to make `=>` completely first class. Feel free to close if you think this has no chance of being implemented any time soon. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9115#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#9115: The kind of (=>) -------------------------------------+------------------------------------ Reporter: kosmikus | Owner: Type: feature request | Status: closed Priority: low | Milestone: Component: Compiler | Version: 7.8.2 Resolution: wontfix | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Changes (by goldfire): * status: new => closed * resolution: => wontfix Comment: I will close, but your post made me realize something. Instead of writing `(Eq a, Show a, Read a) => a -> a`, I can write `Eq a => Show a => Read a => a -> a`, which I somehow like more. Haven't checked how it Haddocks, though... -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9115#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#9115: The kind of (=>) -------------------------------------+------------------------------------ Reporter: kosmikus | Owner: Type: feature request | Status: closed Priority: low | Milestone: Component: Compiler | Version: 7.8.2 Resolution: wontfix | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by Fuuzetsu): It Haddocks in whichever way you wrote it. Whether this is desirable or not is arguable but that's how it is. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9115#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#9115: The kind of (=>) -------------------------------------+------------------------------------ Reporter: kosmikus | Owner: Type: feature request | Status: closed Priority: low | Milestone: Component: Compiler | Version: 7.8.2 Resolution: wontfix | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by ekmett): goldfire: Note: the latter is slightly weaker, since in Eq a => Show a => Read a => .. you can only reference backwards up the chain. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9115#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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