Fix overflows in randomInt

[hdgarrood]

Fixes #9

Fix overflows by turning everything into a Number first, and then converting it back at the end.

I decided not to bother with the export list yet because I thought you might want safeFromNumber to go into purescript-integers instead, perhaps with a different name. (truncate?) What do you think?

[paf31]

This shouldn't be exported, right? Can it go into a where clause?

[paf31]

Sorry, I should read your comments first :)

Yeah, let's put this into integers as truncate. Although, I guess it depends what we decide to do about bounds on integers for different backends.

[hdgarrood]

No, it shouldn't, but I didn't do that yet becaues I thought you might want to put this function into integers instead. I can imagine it being useful in other contexts.

[hdgarrood]

Ah, just saw your second comment!

Regardless of what we do about different backends, top :: Int should still work on any backend, right? Perhaps Prelude can get the maximum and minimum values for the Bounded Int instance from FFI files, so that we can make them differ across backends?

[garyb]

What is this actually fixing? Since high is an Int, isn't it already safe from overflow?

[hdgarrood]

If you ask for randomInt bottom top, then the current code will do:

toNumber (high - low + one) * n + toNumber low
== toNumber (top - bottom + one) * n + toNumber bottom
== toNumber 0 * n + toNumber bottom
== 0 + bottom
== bottom

so randomInt bottom top always evaluates to bottom.

[garyb]

Ah ok :)

[hdgarrood]

Now that integers has been updated and released, I think this is good to go.