A Swift solution for http://rosalind.info/problems/rna/.

rna-solution.swift

#!/usr/bin/xcrun swift -F .
// FileInput module from https://github.com/otto-schnurr/FileInput-swift
import FileInput

func complement(base: Character) -> Character {
    switch base {
        case "T": return "U"
        default: return base
    }
}

let dna = input().nextLine()!
print(String(seq: map(dna, complement)))