type operators and colon in GHC

Alexey Vagarenko vagarenko at gmail.com
Thu Dec 10 15:54:59 UTC 2015

This ticket might be relevant https://ghc.haskell.org/trac/ghc/ticket/11046

2015-12-10 20:51 GMT+05:00 John Leo <leo at halfaya.org>:

> I sent this to haskell-cafe a little while ago but didn't get a response,
> so I thought I'd try here.  I'd guess this is a case of the GHC user guide
> needing an update, but I'd like an expert opinion.
> ---
> According to sections 7.4.3 and 7.4.4 of the latest GHC documentation
> https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/data-type-extensions.html
> you can define (7.4.3) an infix type constructor as long as it begins with
> a colon, for example
> data a :*: b = Foo a b
> and furthermore (7.4.4) you can define an infix operator without having to
> use a colon if you enable the TypeOperators extension:
> data a * b = Foo a b
> However if I try the former without using TypeOperators I get this
> compiler error in 7.10.2:
>     Illegal declaration of a type or class operator ‘:*:’
>       Use TypeOperators to declare operators in type and declarations
> Using TypeOperators fixes this, but then * without colon also works so I
> don't see the point of using colon anymore.
> My guess is this was some some kind of historical distinction which is no
> longer valid and the documentation needs to be updated.  Is this true, or
> am I missing something?
> John
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