Hindley-Milner Type Inference (2012)

Re:Hindley-Milner Type Inference (2012) by roebk on Friday July 10, 2020 @03:43PM (#23795537)

I can’t speak for Ocaml, but in Elm, an if expression as well as a function must return the same type. Therefore the code you posted would be invalid.

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Re:Hindley-Milner Type Inference (2012) by 1-more on Friday July 10, 2020 @05:15PM (#23796559)

To expand slightly, the Elm compiler will help you out here and show you what you need to fix. Given this function body

  randomReturner n =
      if n < 0.5 then
          123
      else
          "abc"

The compiler will give you the following info:

  The 2nd branch of this `if` does not match all the previous branches:

  22|     if n < 0.5 then
  23|         123
  24|     else
  25|         "abc"
            ^^^^^
  The 2nd branch is a string of type:

      String

  But all the previous branches result in:

      number

  Hint: All branches in an `if` must produce the same type of values. This way, no
  matter which branch we take, the result is always a consistent shape. Read
  <https://elm-lang.org/0.19.1/custom-types> to learn how to “mix” types.

  Hint: Try using String.toInt to convert it to an integer?

You can try it out on Ellie here: https://ellie-app.com/9nXxfKSStG4a1

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