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Elm challenge on Exercism: Rna Transcription

I'm gonna be short.

I'm going through exercism exercises.

At one moment I had a beef with the compiler.

The 4th and 5th branches of this if produce different types of values.

4| if x == 'G' then
5| 'C'
6| else if x == 'C' then
7| 'G'
8| else if x == 'T' then
9| 'A'
10| else if x == 'A' then
11| 'U'
12| else
13|> "There is an Unknown Character in the DNA sequence!!!"

The 4th branch has this type:


But the 5th is:


Hint: All the branches of an if need to match so that no matter which one we
take, we get back the same type of value overall.

To me, it seemed unreasonable :)

Now, I'm just stuck here.

The definition of toRNA does not match its type annotation.

17| toRNA : String -> Result Char String
18| toRNA dna =
19|> converted_to_list dna
20|> |> convert
21|> |> String.fromList

The type annotation for toRNA says it always returns:

Result Char String

But the returned value (shown above) is a:


Detected errors in 1 module.

Top comments (8)

drbearhands profile image
DrBearhands • Edited

You have a fundamental representation problem:
you should not use a Char to describe a nucleotide base. Allowed values are only A, C, T and G. With this representation it could also be Z or ;...

Instead, create a new type:

type DNANucleotideBase
  = Adenine
  | Thymine
  | Guanine
  | Cytosine

or you can also use A | T | G | C for brevity if that does not conflict with anything.

That way you will not have to consider incorrect values because they cannot exist.

Also consider using a case x of rather than multiple ifs.

antonrich profile image
Anton • Edited

First, I'm gonna try deal wit the case and then with the Result type and then I will add types.

P.S. I just got my hand on frondendmasters elm courses. I will postpone solving the problem. Want to watch the videos first.

drbearhands profile image

You won't need Result in this case if you make impossible state impossible :-)

BTW, have you tried the official Elm documentation? I'm not sure how they hold up for people new to functional programming but I myself found them sufficient (for the fundamentals anyway).

Thread Thread
stevensonmt profile image

The exercise is written such that it expects to get a Result.

Thread Thread
drbearhands profile image
DrBearhands • Edited

I would argue this isn't a good exercise then. There are better way to explain result, that do not promote improper state representation (such as making a safe division function).

I apologize for being harsh. I understand writing this stuff is time-consuming and sometimes hard, but I do believe this (making impossible state impossible) is an important concept, central to Elm and FP in general.

stevensonmt profile image

Consider rewriting those conditionals as a case statement. Then make the return of the case statement a Result type.

case x of
    'G' -> Just 'C'
    'C' -> Just 'G'
    'T' -> Just 'A'
    'A' -> Just 'U'
    _ -> Err "There is an Unknown Character in the DNA sequence!!!"

Now all arms are the same type and the compiler won't complain.

antonrich profile image

I wanted to use case, but then I wondered if elm has if else expression. So I didn't get to the case.

Thanks for pointing out to the Result type. I didn't know that exists.

stevensonmt profile image

I believe case statements are considered more idiomatic in elm.