#10189: explicit promotions of prefix data constructors can't be parsed naturally -------------------------------------+------------------------------------- Reporter: Kinokkory | Owner: Kinokkory Type: bug | Status: new Priority: normal | Milestone: 7.12.1 Component: Compiler | Version: 7.8.4 (Parser) | Keywords: Resolution: | Architecture: Operating System: Unknown/Multiple | Unknown/Multiple Type of failure: GHC rejects | Test Case: valid program | Blocking: Blocked By: | Differential Revisions: Related Tickets: #10188 | -------------------------------------+------------------------------------- Old description:
When used as an infix operator, a data constructor is explicitly promoted simply with `'` prefixed, but when used as a prefix operator enclosed between `(` and `)`, it is explicitly promoted only with `'` put before `(`, not before the constructor.
On the other hand, messages of GHC and GHCi indicate that a data constructor operator in a prefix form is promoted by putting `'` before the constructor! (as in `forall (k :: BOX) (k :: BOX). (':*)` below.)
I believe that this is a matter of parsing. The parser should admit `(':*)` as well as `'(:*)` for the naturalness of the syntax.
In a GHCi:
{{{#!hs
:set -XDataKinds -XTypeOperators data a :* b = a :* b :kind! Int :* Int Int :* Int :: * = Int :* Int :kind! Int ':* Int Int ':* Int :: * :* * = Int ':* Int :kind! (:*) (:*) :: * -> * -> * = (:*) :kind! '(:*) '(:*) :: k -> k1 -> k :* k1 = forall (k :: BOX) (k :: BOX). (':*) :kind! (':*)
<interactive>:1:3: parse error on input ‘:*’ }}}
(By the way I assume that Template Haskell quotes (`'function` and `''Type`) are parsed by the same mechanism as the one for the explicit promotion syntax. I hope both styles of quotes for prefix operators will be admitted because currently only the `'(` `''(` style is admitted.)
New description: When used as an infix operator, a data constructor is explicitly promoted simply with `'` prefixed, but when used as a prefix operator enclosed between `(` and `)`, it is explicitly promoted only with `'` put before `(`, not before the constructor. On the other hand, messages of GHC and GHCi indicate that a data constructor operator in a prefix form is promoted by putting `'` before the constructor! (as in `forall (k :: BOX) (k :: BOX). (':*)` below.) I believe that this is a matter of parsing. The parser should admit `(':*)` as well as `'(:*)` for the naturalness of the syntax. In a GHCi: {{{#!hs
:set -XDataKinds -XTypeOperators data a :* b = a :* b :kind! Int :* Int Int :* Int :: * = Int :* Int :kind! Int ':* Int Int ':* Int :: * :* * = Int ':* Int :kind! (:*) (:*) :: * -> * -> * = (:*) :kind! '(:*) '(:*) :: k -> k1 -> k :* k1 = forall (k :: BOX) (k :: BOX). (':*) :kind! (':*)
<interactive>:1:3: parse error on input ‘:*’ }}} -- Comment (by Kinokkory): (Removed the mention about Template Haskell syntax.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10189#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler