On Oct 13, 2007, at 13:30 , ntupel wrote:
On Sat, 2007-10-13 at 12:42 -0400, Brandon S. Allbery KF8NH wrote:
Your apparently simple StdGen argument is actually a sort of program
state (represented by unevaluated thunks, not by a state monad; see
below) which gets altered with every invocation of random. If
nothing is forced until the very end, it in effect becomes an
expression which produces the desired StdGen, with the uses of the
previous StdGen values as "side effects" of its computation that
occur when the thunk is evaluated at the end. I'm not sure I'm up to
working through an example of what this looks like.
Thanks Brandon. I understand your argument but I don't know how to put
it into practice, i.e. how to force the evaluation of StdGen.
For starters, look into "seq". Try applying it to any expression
using a generated random number. This should force evaluation to
occur somewhere other than when random is trying to figure out what
StdGen value it's been told to use as its initial state.
Alternately you can put all the uses in IO and use
Control.Exception.evaluate (or even print). This is probably not
what you want to do in your actual production code, however.
--
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED]
system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED]
electrical and computer engineering, carnegie mellon university KF8NH
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