gistfile1.hs

import qualified Data.Map as Map

type Mol = String
type Mixture = Map.Map Mol Integer
type Rxn = (Mixture, (Mixture -> Mixture), Double)

getActivities :: [Rxn] -> Mixture -> [Double]
getActivities rxns mixture =
  let f acc mol sc = if sc == 1 -- sc is stoichiometric coefficient
                       then fromIntegral $ mixture Map.! mol
                       else let n = mixture Map.! mol
                                m = foldl' (*) 1 [n, n-1..n-sc+1] -- possible combinations of molecules mol
                            in acc * (fromIntegral m / fromIntegral sc)
      activity (lhs, _, rate) = Map.foldlWithKey f rate lhs
  in map activity rxns

{- Error:
test.hs:13:54:
    No instance for (Fractional Integer)
      arising from a use of `/'
    Possible fix: add an instance declaration for (Fractional Integer)
    In the second argument of `(*)', namely
      `(fromIntegral m / fromIntegral sc)'
    In the expression: acc * (fromIntegral m / fromIntegral sc)
    In the expression:
      let
        n = fromIntegral $ mixture Map.! mol
        m = foldl (*) 1 [n, n - 1 .. n - sc + 1]
      in acc * (fromIntegral m / fromIntegral sc)

test.hs:15:10:
    Couldn't match expected type `Double' with actual type `Integer'
    Expected type: (Map.Map Mol a0, t0, Integer) -> Double
      Actual type: (Map.Map Mol a0, t0, Integer) -> Integer
    In the first argument of `map', namely `activity'
    In the expression: map activity rxns
-}