Applying a function on an F# array - arrays

Question

type Sizes =
| Big
| Medium
| Small
;;
//defines cup/can/bottle with size
type Containment =
| CupDrink of s:Sizes
| CannedDrink of s:Sizes
| BottledDrink of s:Sizes
;;
// defines record for each type for drink
type Coffee = {DrinkName : string; price: double }
type Soda = {DrinkName : string; price: double }
type Brew = {DrinkName : string; price: double }
// union for type of drink
type liquid =
| Coffee of c:Coffee
| Cola of s:Soda
| Beer of b:Brew
;;
let Guiness = Beer {DrinkName = "Guiness"; price = 0.15}
let CocaCola = Cola {DrinkName = "Cola"; price = 0.15}
let smallCup = CupDrink Small // it could be just containment | size
let bigBottle= BottledDrink Big
let findPricePrML(dr:liquid) =
let price = 0.0
match dr with
|Beer(b=h)-> h.price
|Cola(s=h) ->h.price
|Coffee(c=h) -> h.price
|_-> failwith "not found"
// returns size in ML fro each size available // asuming that small bottle , can and cup have same size,
//if not another kind of program can be made but it's not part of the assingment
let find size =
match size with
|Big -> 250.00
|Medium -> 125.00
|Small -> 75.00
let grandTotal (dr:liquid ,cont:Containment) = function
(*let temp = {dra=dr;conta = cont} //supossed to search on menu list if such item exists (can't figure the syntax)
if List.contains temp menuList then *)
|CupDrink (s=z) -> findPricePrML dr * find z
|BottledDrink (s=z) -> findPricePrML dr * find z
|CannedDrink (s=z) -> findPricePrML dr * find z
|_-> failwith "not found"
(* else failwith "no such item exists"*)
;;
let source = [|(CocaCola, bigBottle); (CocaCola, smallCup); (Tuborg, smallCup)|]
let Test =
Async.Parallel [ async { return Array.map grandTotal source } ]
|> Async.RunSynchronously
Hello, I'm trying to learn basics of CPU bound parallel programming in F#. Here I have a function that calculates drink prices. And all i want is to apply another function(which multiplies the result with a certain number) to the results i get from the parallel calculation but I keep getting type mismatch errors. In my solution the result i get is a jagged array. Unfortunately i couldn't figure out how to get the results just as an array too.

Answer 1

I think that the first issue with your code is that your grandTotal function takes two arguments:
let grandTotal (dr:liquid, cont:Containment) = function
| ...
This means that you have to call it with something like grandTotal (CocaCola, bigBottle) drinkKind. However, in your code that tries to call this, you use:
Array.map grandTotal source
This calls grandTotal with only a single argument - an item from the source list, so you get back a function rather than a price. You probably need something like:
Array.map (fun drink -> grandTotal drink kind) source
The second issue is that you are not really parallelising anything. The way you use async, you are just creating a single computation and then running that on a background thread. You could do something like:
let test =
[ for a in source -> async { return grandTotal a kind } ]
|> Async.Parallel
|> Async.RunSynchronously
However, a more efficient and simpler approach is to use Array.Parallel.map:
Array.Parallel.map (fun drink -> grandTotal drink kind) source
To answer your question about calling another function - this is impossible without seeing a more complete code sample that we can run.