Discussion on: Use opaque types in Elm!

hecrj profile image
Héctor Ramón Author

Then the create function does multiple things. Not desirable. For instance, how would you show validation errors without submitting the form?

steppeeagle profile image

Then the create function does multiple things

For instance we have function which get 2 numbers and return sum of them. The first number is odd and the second is even.

function oddEvenSumm(odd, even) {
  if (even % 2 == 1) {
    throw new Error('not even one')
  if (odd % 2 == 0) {
    throw new Error('not odd one')

  return odd + even;

Should we create new types for these values (like you did for title and body) because we validate them?
Do we break Single Responsobiliy Principal?
Does it make sence to create separate type if we don't use them in other places?

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hecrj profile image
Héctor Ramón Author

Opaque types allow to capture guarantees and propagate them. If you only need these guarantees on a specific part of your code and there is no need to propagate them, then it is probably not worth it to create opaque types for them. In other words, not every "validation check" should result in an opaque type.

However, most of the time this is an API design choice. For instance, I would understand this API better than the example you provided:

type Odd = Odd Int
type Even = Even Int

oddFromInt : Int -> Maybe Odd
evenFromInt : Int -> Maybe Even
sum : Odd -> Even -> Odd

The advantages here are:

  • We move potential errors closer to their cause (conversions).
  • We keep sum error-free! We can call it multiple times without having to deal with errors: sum (sum odd even) even
  • We specify that the returned sum value is guaranteed to be an Odd number.