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to RNA Transcription in the Elm Track

Published at Apr 19 2021 · 0 comments
Test suite

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:

  • G -> C
  • C -> G
  • T -> A
  • A -> U

Elm Installation

Refer to the Installing Elm page for information about installing elm.

Writing the Code

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:

$ elm-test

To automatically run tests again when you save changes:

$ elm-test --watch

As you work your way through the tests suite in the file tests/Tests.elm, be sure to remove the skip <| calls from each test until you get them all passing!


Hyperphysics http://hyperphysics.phy-astr.gsu.edu/hbase/Organic/transcription.html

Submitting Incomplete Solutions

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 _ ->

        Err _ ->

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)

toRNA : String -> Result String String
toRNA dna =
        translated =
                |> String.toList
                |> List.map translateChar
    if List.any isError translated then
        Err "Invalid input."

        Ok (String.fromList <| List.map unwrap translated)

unwrap : Result error Char -> Char
unwrap res =
    case res of
        Ok c ->

        Err _ ->
            ' '

isError : Result error value -> Bool
isError res =
    case res of
        Ok _ ->

        Err _ ->

translateChar : Char -> Result String Char
translateChar c =
    case c of
        'C' ->
            Ok 'G'

        'G' ->
            Ok 'C'

        'T' ->
            Ok 'A'

        'A' ->
            Ok 'U'

        _ ->
            Err "cannot translate"

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