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.
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.
A list can store zero or more elements. A NonEmpty can store one or more element. That’s all.
This overall strategy – representing the top of a list as a dedicated value – shows up elsewhere, notably in Forths, where it is called “top of stack” and often stored in a dedicated CPU register.
So it’s the implementation that has to ensure a NonEmpty is returned, but that’s up to the developer, correct? The developer still holds the gun to shoot themselves in the foot by returning an empty list, IINM.
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.
Anti Commercial-AI license
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.A list can store zero or more elements. A
NonEmptycan store one or more element. That’s all.This overall strategy – representing the top of a list as a dedicated value – shows up elsewhere, notably in Forths, where it is called “top of stack” and often stored in a dedicated CPU register.
You cannot, and that’s why that type declaration models a
NonEmptythat a type checker can enforceSo it’s the implementation that has to ensure a
NonEmptyis returned, but that’s up to the developer, correct? The developer still holds the gun to shoot themselves in the foot by returning an empty list, IINM.Anti Commercial-AI license