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0 pointsedflsafoiewq6y ago0 comments

Pop an item from an array. What do you return when the array is empty?

Solution: nil. Problem: How do you tell the difference between "the array was empty" and "the item you popped was nil"?

Solution: have pop return a Maybe instead.

0 comments

or allow returning multiple values. for example in CL searching for a thing returns two values the is-found and the value

uryga6y ago

...which, btw, is equivalent to Maybe/Either – they're tagged unions, i.e. pairs (tag, content). Maybe<T> is isomorphic* to

  { has_value: bool,
    val: T|nil }

* well, this representation is a bit too permissive, since you could do

  {has_value: true, val: nil}

if you wanted to get the types water-tight, you'd need dependent types, typing it as dependent pair:

  Maybe<T> =
    sigma (has_value: bool)
      if has_value
        then T
        else ()

which can then only have values

  (false, ())

  (true, <actual value of type T>)

Which is the same thing, but in an ad-hoc way, and leaves it open to the developer to check or not, and thus to crash.

or solution: Null Object

or a whole value

or use an enumerator

or a collection decorator

All of which are simple solutions already available. No need to import every new paradigm hammer in the hope that everything is a nail.

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