System.Random.Mersenne - no randomR variants
I notice that System.Random.Mersenne has no randomR variants. Is there a reason for this, or is it just an oversight? It makes porting code from System.Random a little more complicated than it needs to be. I know it's possible to write a function mapping a Double (or other value, as appropriate) to a range, effectively replacing randomR, but it's fiddly to get the edge cases right. A standard implementation would be helpful. Paul
p.f.moore:
I notice that System.Random.Mersenne has no randomR variants. Is there a reason for this, or is it just an oversight? It makes porting code from System.Random a little more complicated than it needs to be. I know it's possible to write a function mapping a Double (or other value, as appropriate) to a range, effectively replacing randomR, but it's fiddly to get the edge cases right. A standard implementation would be helpful.
Basically, to use the type class interface defined in System.Random, you will suffer a 100x decrease in performance, as the instances all convert via Integer. Rather than give people a surprise, I chose not to implement that interface. -- Don
2009/10/5 Don Stewart
p.f.moore:
I notice that System.Random.Mersenne has no randomR variants. Is there a reason for this, or is it just an oversight? It makes porting code from System.Random a little more complicated than it needs to be. I know it's possible to write a function mapping a Double (or other value, as appropriate) to a range, effectively replacing randomR, but it's fiddly to get the edge cases right. A standard implementation would be helpful.
Basically, to use the type class interface defined in System.Random, you will suffer a 100x decrease in performance, as the instances all convert via Integer.
Rather than give people a surprise, I chose not to implement that interface.
Sounds fair enough :-) Excuse my ignorance, but although System.Random uses Integer, why can't your implementation work with Int and other fast types? Is this something to do with how classes work? Also, a minor documentation question - the documentation for getStdRandom seems confused (or I am!): getStdRandom :: (MTGen -> IO a) -> IO a Source Uses the supplied function to get a value from the current global random generator, and updates the global generator with the new generator returned by the function. For example, rollDice gets a random integer between 1 and 6: rollDice :: IO Int rollDice = getMTRandom (randomR (1,6)) Is this right? It seems to imply there is a way to use randomR. If I did, would performance be impacted as you mention above? Thanks, and sorry for the dumb questions. Paul.
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Don Stewart -
Paul Moore