type operators and colon in GHC

John Leo leo at halfaya.org
Thu Dec 10 15:58:52 UTC 2015

Thanks, that's helpful.  Sounds like the situation is even more complicated
than I realized.  It will be great if the documentation can be updated once
the behavior stabilizes.


On Thu, Dec 10, 2015 at 7:54 AM, Alexey Vagarenko <vagarenko at gmail.com>

> 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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