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.nethaskellf#monadscomputation-expression

Re-assign a variable in a function with computation expression

Based on the question How can I re-assign a variable in a function in Haskell?, there is a haskell solution to change the total congo elephant count Congo 0 in a function:

main' :: StateT Congo IO ()
main' =
  do
    printElephant
    function 2
    printElephant

-- run program:
main :: IO ()
main = Congo 0 & runStateT main' & void
-- outputs:
0
2

After reading the Computation Expression series, I still don't know how to write a CE builder for this problem correctly. How can I re-assign a variable in a function with F#'s CE?

Solution

  • F# has a first-class support for imperative programming constructs. You just need to mark your let bindings as mutable. There is no need for computation expressions in this case:

    let mutable elephant = 0
printfn "Elephant = %d" elephant
elephant <- 2
printfn "Elephant = %d" elephant