• QT1@lemm.ee
    15·
    3 days ago

    I’ve first read this post back in 2019 when it was released and I have to say that it really has left quite an impact on the way I write programs these days. The „make illegal states unrepresentable“ and „push proofs up“ guidelines are so simple yet so effective. Sure, there is some initial cost to create new datatypes, but it really pays off in the long run. Not having to worry about null or wrongly shaped data structures down the line is really nice, especially if you’re working on older code or develop in a team. Even though the post uses Haskell to explain the concepts, I found it to also work well in other languages, even Java or Python.

  • livingcoder@programming.dev
    19·
    4 days ago

    This was a good blog post. I particularly appreciated the statement about the validate and parse function comparison: “Both of these functions check the same thing, but parseNonEmpty gives the caller access to the information it learned, while validateNonEmpty just throws it away.”

    • QT1@lemm.ee
      9·
      3 days ago

      void in Java and Void in Haskell are quite different. As the post explains, in Haskell it’s a type with no possible values. In Java, the equivalent would be a class without a constructor (not sure if that’s even possible). It defines a type, but you cannot construct a value or object with that type. The equivalent of Java‘s void in Haskell is the unit type () which has exactly one possible value, also called (). It can be returned by a function, but it does not give you any information, just like void. By the way, Rust also uses the unit type instead of void.

  • onlinepersona@programming.devEnglish
    2·
    3 days ago
    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.

    Anti Commercial-AI license

    • Ephera@lemmy.mlEnglish
      2·
      3 days 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.

    • Corbin@programming.devEnglish
      3·
      3 days ago

      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.

    • Kache@lemm.ee
      2·
      3 days ago

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