Re: [GHC] #10794: Extension request: "where" clauses in type signatures

25 Aug 2015

      #10794: Extension request: "where" clauses in type signatures
-------------------------------------+-------------------------------------
        Reporter:  danso             |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  lowest            |               Milestone:
       Component:  Compiler (Type    |                 Version:  7.10.2
  checker)                           |
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
-------------------------------------+-------------------------------------
Description changed by danso:

Old description:
...
Some type signatures could be simplified by separating certain elements,
either because they are lengthy and can be expressed simpler, to better
express intention, or because they  are repetitive. A "WhereSignatures"
extension would allow for type signatures like the following:
{{{#!hs
fold :: f -> [a] -> a
        where f :: a -> a -> a
-- expresses intention more clearly than:
fold :: (a -> a -> a) -> [a] -> a
union :: a -> a -> a
         where a :: Map.Map k v
-- much shorter/more readable than:
union :: Map.Map k v -> Map.Map k v -> Map.Map k v
}}}
Or maybe even:
{{{#!hs
fmap :: (a -> b) -> f a -> f b
        where (Functor f)
-- arguably prettier than:
fmap :: (Functor f) => (a -> b) -> f a -> f b
}}}
This minor syntactic sugar is essentially equivalent to local "type"
definitions, which provides an advantage of making the type's purpose
more clear. This seems like a natural development on what the type
checker can do.
New description:

 Some type signatures could be simplified by separating certain elements,
 either because they are lengthy and can be expressed simpler, to better
 express intention, or because they  are repetitive. A "WhereSignatures"
 extension would allow for type signatures like the following:

 {{{#!hs
 fold :: f -> [a] -> a
         where f = a -> a -> a

 -- expresses intention more clearly than:
 fold :: (a -> a -> a) -> [a] -> a

 union :: a -> a -> a
          where a = Map.Map k v

 -- much shorter/more readable than:
 union :: Map.Map k v -> Map.Map k v -> Map.Map k v
 }}}

 Or maybe even:

 {{{#!hs
 fmap :: (a -> b) -> f a -> f b
         where (Functor f)

 -- arguably prettier than:
 fmap :: (Functor f) => (a -> b) -> f a -> f b
 }}}

 This minor syntactic sugar is essentially equivalent to local "type"
 definitions, which provides an advantage of making the type's purpose more
 clear. This seems like a natural development on what the type checker can
 do.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10794#comment:2
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