FSharp http://rosalind.info/problems/dna/

nucleotidecount.fs

open System.Linq

[<EntryPoint>]
let main argv = 

    let counter = List.fold (fun (a,c,g,t) x -> match x with 
                                                    | 'A' -> (a+1, c,   g,   t  )
                                                    | 'C' -> (a,   c+1, g,   t  )
                                                    | 'G' -> (a,   c,   g+1, t  )
                                                    | 'T' -> (a,   c,   g,   t+1)
                                                    | ___ -> failwith "unknown nucleotide")  (0,0,0,0)

    let atot, ctot, gtot, ttot = counter [for c in argv.First().ToUpper() -> c]
    printfn "%d %d %d %d" atot ctot gtot ttot
    
    0 // return an integer exit code