[GHC] #11628: Unexpected results with Read/Show
GHC
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Mon Feb 22 21:22:58 UTC 2016
#11628: Unexpected results with Read/Show
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Reporter: crockeea | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.10.3
Resolution: | Keywords:
Operating System: Unknown/Multiple | Architecture:
Type of failure: Incorrect result | Unknown/Multiple
at runtime | Test Case:
Blocked By: | Blocking:
Related Tickets: | Differential Rev(s):
Wiki Page: |
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Comment (by rwbarton):
Lots of distractions here. Here is the crux of the matter:
{{{
newtype X u = X u
instance Show u => Show (X u) where show (X u) = show u
main = print (Just (X (Just 1))) -- prints "Just Just 1"
}}}
Of course, you should just define `showsPrec` like you mentioned.
Just defining `show` should be okay if (either you don't care about
compatibility with Read or) the output is a single token, like a number or
a quoted string. Otherwise you need to define `showsPrec`. Agree that the
docs should be clearer on this point.
--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11628#comment:2>
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