example.nim

import sequtils
from strutils import join

proc toRna*(dna: string): string =
  result = toSeq(dna)
    .map(c => 
      case c:
        of 'A':
          return 'U'
        of 'T':
          return 'A'
        of 'C':
          return 'G'
        of 'G':
          return 'C'
        else:
          raise newException(ValueError, "invalid DNA character detected")
    ).join()