#5279: numericEnumFromThen breaks standard behaviour in presence of ⊥
---------------------------------+------------------------------------------
Reporter: ehird | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.0.3 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: Incorrect result at runtime
---------------------------------+------------------------------------------
Haskell 2010 (and, for what it's worth, 98) require the following
behaviour for `numericEnumFromThen`:
{{{
numericEnumFromThen n m = iterate (+(m-n)) n
}}}
but GHC has:
{{{
numericEnumFromThen n m = n `seq` m `seq` (n : numericEnumFromThen m
(m+m-n))
}}}
The standard evaluation of `numericEnumFromThen 0.0 undefined` is
equivalent to `0.0 : repeat undefined`, but GHC evaluates it as
`undefined`:
{{{
> Prelude> [0.0, undefined..]
*** Exception: Prelude.undefined
}}}
So unfortunately, this optimisation changes semantics in violation of the
Report.
`numericEnumFrom` also suffers from this issue; the evaluation of
`numericEnumFrom undefined` should be equivalent to `repeat undefined`,
but GHC evaluates it as `undefined`. This is a more minor issue, however.
:)
(Tested with 7.0.3 and whatever version lambdabot uses on freenode.)
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5279>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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