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Parse, don’t validate (lexi-lambda.github.io)

submitted 3 months ago by ephemera3444@lemmy.blahaj.zone to c/programming@programming.dev

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[–] onlinepersona@programming.dev 1 points 3 months ago (2 children)

data NonEmpty a = a :| [a]

Note that NonEmpty a is really just a tuple of an a and an ordinary, possibly-empty [a]. This conveniently models a non-empty list by storing the first element of the list separately from the list’s tail: even if the [a] component is [], the a component must always be present.

Wat? How can I "store the first element of the list separated from the lists tail" when the list is empty? Whether a list is empty or not is a runtime possibility, not a compile-time possibility.

Someone care to explain this part? It does not compute at all for me.

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[–] Kache@lemm.ee 1 points 3 months ago

You cannot, and that's why that type declaration models a NonEmpty that a type checker can enforce

[–] Ephera@lemmy.ml 1 points 3 months ago

During the parsing step, you check that the list has at least one element. If it does not, you report an error to the user and exit. If it does, you take the first element in the least and store it in the left side of your tuple, and then the remaining elements of the input list go into the right side of your tuple.

So, for example: [1, 2, 3] → (1, [2, 3])
Or also: [1] → (1, [])
If the user gives you [], then you cannot represent that with your tuple, you necessarily have to error.