Bjorn Lisper wrote:
Chris Kuklewicz:
Stefan Karrmann wrote:
Dear all,
can ghc compile huge tables into efficient code if they are constant at
compile time?
Two examples may clearify the question:
big1 :: UArray Int Char
big1 = array (0,1000) $! map (\i -> (i,toEnum i)) [0..1000]
big2 = sum [0..10000]::Int -- == 50005000 == n*(n+1)/2 where n = 10000
GHC does not compute such values are compile time because
*) The computation may not finish in reasonable time (infinite/not halting)
*) The computation may generate a run-time error and crash the compilation
(divide by zero, pattern march failure, etc.)
*) Haskell is supposed to be lazy, things are computed when needed, not before
Does this mean that GHC does not evaluate constant subexpressions at
compile-time?
No, constant folding is definitely one of the optimisations that GHC
performs.
Or does it evaluate only some subclass of surely terminating
and non-erroneous subexpressions at compile-time?
Yes, the compiler will not evaluate expressions that raise exceptions or
fail. For example, integer division is only performed at compile time
for a non-zero dividend.
Cheers,
Simon
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