ComplementingAStrandOfDNA.hs

import Bioinformatics.DNANucleotide as D

main = do
  substrand <- getLine
  putStrLn . concat . map show . reverse . map D.nucleotideComplement $ map D.charToDNANucleotide substrand

DNANucleotide.hs

module Bioinformatics.DNANucleotide (
  DNANucleotide (..),
  charToDNANucleotide,
  nucleotideComplement
) where

import Data.Char
import Data.List

data DNANucleotide = A | C | G | T deriving(Show, Eq, Ord, Read)

charToDNANucleotide :: Char -> DNANucleotide
charToDNANucleotide c = read [toUpper c] :: DNANucleotide

nucleotideComplement :: DNANucleotide -> DNANucleotide
nucleotideComplement A = T
nucleotideComplement T = A
nucleotideComplement C = G
nucleotideComplement G = C