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.

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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:

Char

But the 5th is:

String

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.

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To me, it seemed unreasonable :)

Now, I'm just stuck here.

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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|> |> List.map convert
21|> |> String.fromList

The type annotation for toRNA says it always returns:

Result Char String

But the returned value (shown above) is a:

String

Detected errors in 1 module.

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Comments

[DrBearhands]

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.

[Anton]

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]

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).

[stevensonmt]

The exercise is written such that it expects to get a Result.
github.com/exercism/elm/blob/maste...

[DrBearhands]

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]

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.

[Anton]

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]

I believe case statements are considered more idiomatic in elm.