[GHC] #11601: Strict Haskell is not as strict as it probably should be

[GHC]

#11601: Strict Haskell is not as strict as it probably should be
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        Reporter:  simonpj           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"e3f341f334d89c88f388d8e864ed8762d0890a64/ghc" e3f341f/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="e3f341f334d89c88f388d8e864ed8762d0890a64"
 Fix and refactor strict pattern bindings

 This patch was triggered by Trac #11601, where I discovered that
 -XStrict was really not doing the right thing. In particular,

   f y = let !(Just x) = blah[y] in body[y,x]

 This was evaluating 'blah' but not pattern matching it
 against Just until x was demanded.  This is wrong.

 The patch implements a new semantics which ensures that strict
 patterns (i.e. ones with an explicit bang, or with -XStrict)
 are evaluated fully when bound.

 * There are extensive notes in DsUtils:
   Note [mkSelectorBinds]

 * To do this I found I need one-tuples;
   see Note [One-tuples] in TysWiredIn

 I updated the user manual to give the new semantics
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11601#comment:8>
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