Given a DNA strand, return its RNA complement (per RNA transcription).
Both DNA and RNA strands are a sequence of nucleotides.
The four nucleotides found in DNA are adenine (A), cytosine (C), guanine (G) and thymine (T).
The four nucleotides found in RNA are adenine (A), cytosine (C), guanine (G) and uracil (U).
Given a DNA strand, its transcribed RNA strand is formed by replacing each nucleotide with its complement:
Refer to the Installing Elm page for information about installing elm.
The code you have to write is located inside the
src/ directory of the exercise.
Elm automatically installs packages dependencies the first time you run the tests
so we can start by running the tests from the exercise directory with:
To automatically run tests again when you save changes:
$ elm-test --watch
As you work your way through the tests suite in the file
be sure to remove the
calls from each test until you get them all passing!
It is possible to submit an incomplete solution so you can see how others have completed the exercise.
module Tests exposing (tests) import Expect import RNATranscription exposing (toRNA) import Test exposing (..) isErr : Result error value -> Bool isErr result = case result of Ok _ -> False Err _ -> True tests : Test tests = describe "RNATranscription" [ test "complement of cytosine is guanine" <| \() -> Expect.equal (Ok "G") (toRNA "C") , skip <| test "complement of guanine is cytosine" <| \() -> Expect.equal (Ok "C") (toRNA "G") , skip <| test "complement of thymine is adenine" <| \() -> Expect.equal (Ok "A") (toRNA "T") , skip <| test "complement of adenine is uracil" <| \() -> Expect.equal (Ok "U") (toRNA "A") , skip <| test "complement" <| \() -> Expect.equal (Ok "UGCACCAGAAUU") (toRNA "ACGTGGTCTTAA") , skip <| test "input \"INVALID\" should produce an error" <| \() -> Expect.true "expected an error message output. For example `Err \"Invalid input\"`" (toRNA "INVALID" |> isErr) ]
module RNATranscription exposing (toRNA) -- We could implement the solution using a fold: -- -- toRNA : String -> Result String String -- toRNA = -- String.foldr -- (\char maybeString -> -- Maybe.map2 -- String.cons -- (charToRnaNucleotide char) -- maybeString -- ) -- (Just "") -- >> Result.fromMaybe "Invalid input" -- -- For the purposes of this exercise it'd be just fine! -- However, if we had another requirement where we'd need -- to process very long strands, then that solution -- wouldn't be optimal. When it encounters an invalid -- character, it keeps folding over the strand anyway. -- -- So I wanted to write a solution that stops processing -- the strand at the first sight of an invalid character. -- In Elm that involves writing a recursive function and -- is a bit complicated. -- -- There's a great post that goes into detail how -- languages like Elm handle "loops" from which one would -- wish to break out of at some point: -- http://folkertdev.nl/blog/loops-to-folds/ toRNA : String -> Result String String toRNA strand = toRNAhelper (Just "") strand |> Result.fromMaybe "Invalid input" toRNAhelper : Maybe String -> String -> Maybe String toRNAhelper maybeAcc rest = case maybeAcc of Nothing -> Nothing Just acc -> case String.uncons rest of Nothing -> maybeAcc Just ( char, newRest ) -> let newAcc = char |> charToRnaNucleotide |> Maybe.map String.fromChar |> Maybe.map ((++) acc) in toRNAhelper newAcc newRest charToRnaNucleotide : Char -> Maybe Char charToRnaNucleotide char = case char of 'G' -> Just 'C' 'C' -> Just 'G' 'T' -> Just 'A' 'A' -> Just 'U' _ -> Nothing
I decided to impose another requirement on myself: the function should stop processing the strand at the first sight of an invalid character.
It made the solution much more complicated, but I feel like it was worthwhile to investigate how such a solution could look like!