Compile-time constants could be handled by simple top-level bindings.  This technique is specifically for the case you are after:

mcSimulate :: Double -> Double -> Word64 -> [Double]
mcSimulate startStock endTime seedForSeed = go seedForSeed
 where
    go = fst expiryStock : go newSeedForSeed
      where
     expiryStock = iterate evolveUnderlying (startStock, ranq1Init seedForSeed)
                        !! truncate (endTime/timeStep)
     newSeedForSeed = seedForSeed + 246524

See what's going on there?

I don't know about that nested where.  In Real Life I would probably use a let instead for expiryStock and newSeedForSeed.

Luke

Ahh, I get it now, that’s pretty neat - ‘go’ is only updating the seedForSeed and the expiryStock, the inner ‘where’ clause keeps everything else constant each time it is called.