[GHC] #14367: Lazy evaluation can be invalidated

[GHC]

#14367: Lazy evaluation can be invalidated
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        Reporter:  vanto             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by AntC):

 Replying to [ticket:14367 vanto]:
 > Evaluating arguments only if and when they are needed. This is the first
 principle of lazy evaluation.\\
 > Example:\\
 > ...\\
 > These results are all correct because the evaluated expression
 (function) is non-strict.\\

 No, not an accurate statement. Those examples are ''both'' type-correct
 ''and'' evaluate a non-strict function.

 >
 > The same expression below with a changed argument:\\
 > ...\\

 That example is not type-correct.

 > Here the error message should not have priority. \\

 Type errors ''always'' "have priority" before evaluating an expression.
 (Except if you set `-fdefer-type-errors` or `-fdefer-typed-holes`, as
 @goldfire correctly points out.)

 GHC is chiefly a compiler, not an interpreter. Haskell is a statically-
 typed language. Compiled code would be unusable in general if GHC accepted
 type-incorrect code, then delayed until run-time to see whether it needed
 to evaluate some type-incorrect expression. Programs would run ok today
 but crash tomorrow.

 I think you'll find every statically-typed compiled language behaves like
 that. It really has nothing to do with lazy evaluation.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14367#comment:7>
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