Redundant constraints

[Simon Peyton Jones]

Ha ha ha. Very good, yes!

|  -----Original Message-----
|  From: Michael Sloan [mailto:mgsloan at gmail.com]
|  Sent: 08 January 2015 00:06
|  To: Johan Tibell
|  Cc: Simon Peyton Jones; Milan Straka; Bill Mitchell
|  (Bill.Mitchell at hq.bcs.org.uk); Ross Paterson; ghc-devs at haskell.org
|  Subject: Re: Redundant constraints
|  
|  One option for avoiding the warning without runtime overhead would be
|  to do something like this:
|  
|        f :: Ord a => a -> a -> Bool
|       f x y = True
|         where
|           _ = x < y
|  
|  
|  On Wed, Jan 7, 2015 at 7:27 AM, Johan Tibell <johan.tibell at gmail.com>
|  wrote:
|  > I think this probably makes sense, especially since you can silence
|  > the warning when you intend to add an unnecessary constraint.
|  >
|  > I had one thought though: consider an abstract data type with
|  > functions that operates over it. I might want to require e.g Ord in
|  > the definition of a function so I have freedom to change my
|  > implementation later, even though the current implementation doesn't
|  > need Ord. Think of it as separating specification and
|  implementation.
|  > An example is 'nub'. I initially might implement it as a O(n^2)
|  > algorithm using only Eq, but I might want to leave the door open to
|  > using Ord to create something better, without later having to break
|  backwards compatibility.
|  >
|  > On Wed, Jan 7, 2015 at 4:19 PM, Simon Peyton Jones
|  > <simonpj at microsoft.com>
|  > wrote:
|  >>
|  >> Friends
|  >>
|  >> I’ve pushed a big patch that adds –fwarn-redundant-constraints (on
|  by
|  >> default).  It tells you when a constraint in a signature is
|  >> unnecessary, e.g.
|  >>
|  >>      f :: Ord a => a -> a -> Bool
|  >>
|  >>      f x y = True
|  >>
|  >> I think I have done all the necessary library updates etc, so
|  >> everything should build fine.
|  >>
|  >> Four libraries which we don’t maintain have such warnings (MANY of
|  >> them in
|  >> transformers) so I’m ccing the maintainers:
|  >>
|  >> o   containers
|  >>
|  >> o   haskeline
|  >>
|  >> o   transformers
|  >>
|  >> o   binary
|  >>
|  >>
|  >>
|  >> Enjoy!
|  >>
|  >>
|  >>
|  >> Simon
|  >>
|  >>
|  >> _______________________________________________
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|  >> ghc-devs at haskell.org
|  >> http://www.haskell.org/mailman/listinfo/ghc-devs
|  >>
|  >
|  >
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