[GHC] #15687: Type synonym unused binds no warning?
#15687: Type synonym unused binds no warning? -------------------------------------+------------------------------------- Reporter: AntC | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.6.1 (Parser) | Keywords: | Operating System: Windows Architecture: | Type of failure: GHC accepts Unknown/Multiple | invalid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I was taken aback this was accepted {{{#!hs type Silly a b = Maybe b x :: Silly Int Bool x = Just True y :: Silly Char Bool y = Just False z = x == y -- returns False }}} What's with the `a` in `Silly`'s decl? That's not a phantom type. It's ignored and thrown away. So `x` and `y` are the same type, and can be compare for equality. I expected a rule: all tyvars in the `type`'s head must appear on rhs. Or at least a warning there was something silly. I tried `-Wall, -fwarn- unused-binds`, `-Wunused-type-patterns`. I was just checking up on [https://github.com/ghc-proposals/ghc- proposals/pull/173#issuecomment-424980779 a remark] that type synonyms are at the type level like implicit bidirectional pattern synonyms. For those, all vars must appear on both sides. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15687 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#15687: Type synonym unused binds no warning? -------------------------------------+------------------------------------- Reporter: AntC | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.6.1 (Parser) | Resolution: | Keywords: Operating System: Windows | Architecture: Type of failure: GHC accepts | Unknown/Multiple invalid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): I'm not sure why you think that `type` should introduce a ''pattern'' synonym (which, indeed, must mention all its variables on the right-hand side). It just introduces a (not necessarily injective) synonym. It's true that `-XTypeSynonymInstances` allows you to use a type synonym in an instance head. This might suggest the pattern-like behavior you seek. But this works via the expansion of the synonym, meaning that a program with both `instance C (Silly Int Bool)` and `instance C (Silly Char Bool)` will be rightly rejected. Note that this behavior is not new. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15687#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#15687: Type synonym unused binds no warning? -------------------------------------+------------------------------------- Reporter: AntC | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.6.1 (Parser) | Resolution: | Keywords: Operating System: Windows | Architecture: Type of failure: GHC accepts | Unknown/Multiple invalid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): You could argue that you should get a warning about "unused `a`" in `Silly`'s definition. But we don't warn about {{{ const x y = x }}} so you might consider it inconsistent to warn about the same thing at the type level. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15687#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#15687: Type synonym unused binds no warning? -------------------------------------+------------------------------------- Reporter: AntC | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.6.1 (Parser) | Resolution: | Keywords: Operating System: Windows | Architecture: Type of failure: GHC accepts | Unknown/Multiple invalid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by AntC):
Note that this behavior is not new.
Indeed. Probably been there forever: there's nothing in the Report requiring all type vars to appear on rhs. I can't see there would ever be any sense in it, though; unlike unused vars in other places, for which there's `-fwarn-unused-binds`.
we don't warn about ...
There's sense to not using all the arguments at term level. Never the less it's sufficiently unusual for GHC to recognise a special form. You can (I usually do) write {{{#!hs const x _ = x const x _y = x }}} (Yes I could use `_a` for `Silly`, but I can still see no sense to it.) In some contexs, using `_` avoids getting a warning. (I'm thinking this for `type` could be a small tweak/enhancement for somebody starting GHC development: steal the logic from checking bidirectional patterns.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15687#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#15687: Type synonym unused binds no warning? -------------------------------------+------------------------------------- Reporter: AntC | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.6.1 (Parser) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC accepts | Unknown/Multiple invalid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by Phyx-): * os: Windows => Unknown/Multiple Comment: Not Windows specific, reclassifying. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15687#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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