Combining multiple functions

问题

I'm trying to make a DNA transcription program but I'm having trouble with the way I'm doing it, I'm sure there's an easier way to do this but this was the first thing that came to my head but it's not working the way I want it to.

dnaToRna :: [Char] -> [Char]
dnaToRna [] = []
dnaToRna xs = reverse(transc xs)
    where transc = (replaceA . replaceT . replaceC . replaceG)
replaceA = map(\c -> if c == 'A' then 'U' else c)
replaceT = map(\c -> if c == 'T' then 'A' else c)
replaceC = map(\c -> if c == 'C' then 'G' else c)
replaceG = map(\c -> if c == 'G' then 'C' else c)

Here's the output:

*Main> let seq = "AAATGTTAGTACACTAAGG"
*Main> dnaToRna seq
"GGUUUGUGUUGUUUGUUUU"

I figure this is because the transc replaces the A, then checks the whole String and replaces the T, etc etc Any tips? Thanks in advance!

回答1:

You should make one function which handles all of the Char -> Char conversions at once.

dnaToRna :: [Char] -> [Char]
dnaToRna = reverse . map transc
  where
    transc 'A' = 'U'
    transc 'T' = 'A'
    transc 'C' = 'G'
    transc 'G' = 'C'
    transc _   = error "Invalid DNA molecule"

To make this even safer, you could make it return a Maybe [Char] instead. The lookup function can also be used instead of using a custom mapping function.

dnaToRna :: [Char] -> Maybe [Char]
dnaToRna = mapM (`lookup` zip "ATCG" "UAGC") . reverse

回答2:

@4castle's answer shows the right direction.

Since OP's problem has been solved i believe it would worth to show how to make it more efficient by using Data.Map for the look ups. Also by using a foldl we may skip the reversing job as well.

import qualified Data.Map.Lazy as M

dna2rna :: String -> String
dna2rna = foldl (\r c -> (M.findWithDefault '-' c m) : r) ""
          where m  = M.fromList $ zip "ATCG" "UAGC"

*Main> dna2rna "AAATGTTAGTACACTAAGG"
"CCUUAGUGUACUAACAUUU"

来源：https://stackoverflow.com/questions/47146630/combining-multiple-functions

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haskell