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Comment by muglug | Hacker News Reader

yes, that would be a straightforward error message!

But I got the error message

    Error: This expression has type
         ((locl_ty * Tast.pos * ('ex, 'fb, 'en) Aast.expr_) list ->
          Result_set.t)
         list
       but an expression was expected of type Result_set.t list
       Type
         (locl_ty * Tast.pos * ('ex, 'fb, 'en) Aast.expr_) list ->
         Result_set.t
       is not compatible with type Result_set.t

Which is not half as readable.

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With OCaml type system permanently burned in my mind, I parse that error message as "there is an ` _ -> Result_set.t ` arrow type which is not compatible with type `Result_set.t`". Do you think that the issue is that the arrow `->` is too hard to spot?

Yes, I think that's the issue.

I built a reasonably popular typechecker for PHP, so I have some experience with the DX for these sorts of programs. I don't think it's good for a couple of ASCII characters and line breaks to be the difference between an acceptable and an unacceptable type annotation. It requires, as you say, for the OCaml type system to be burned into people's minds for them to immediately spot the issue.

In a similar context Rust will just say "function expected 5 arguments, you gave it 4" which has a very obvious remediation.

I think it would be great to address this as you describe, but at the same time as somebody who has dabbled in OCaml but is still very wet behind the ears, I don't understand how this error format requires the type system to be "burned into your mind".

I have no insight into what is hard and what is easy to implement here, but might it be feasible to break up the error into parts, the first just as you wrote? Along the lines of:

1. Error: This expression has type ('x -> Result_set.t) list but an expression was expected of type Result_set.t list

2. where 'x = (locl_ty * Tast.pos * ('ex, 'fb, 'en) Aast.expr_) list

The solution suggested by the parent of your post sounds desirable also, but perhaps the currying semantics of Ocaml makes that difficult or poorly-defined?

Emphasizing the arrow (or in fact the most narrow error) is definitively a good idea in that case. Counting the number of arguments is in general a bit problematic: it is possible to end with an accidental functional value due to an extra argument send to a sufficiently polymorphic function for instance.

It might help to make the difference more obvious by also showing the parts which unify properly, or introduce alias variables for complex type expressions (for instance "(int * float array) list as 'a") to move away some of the noise in the error.

For comparison, in Haskell you would get for this definition:

    myfun :: Int -> Char -> Bool
    myfun a b = undefined

if you give too many arguments:

    myfun 1 'c' 3

    • Couldn't match expected type ‘t0 -> t’ with actual type ‘Bool’
    • The function ‘myfun’ is applied to three value arguments,
        but its type ‘p10 -> Char -> Bool’ has only two

if you give too few arguments:

    putStrLn (myfun 1)

    • No instance for (Show (p20 -> Bool))
        arising from a use of ‘print’
        (maybe you haven't applied a function to enough arguments?)

I like how it says

    applied to three value arguments ... but its type ... has only two

and

    maybe you haven't applied a function to enough arguments?

Elm's developers have been working hard to make better error messages for these cases, and it really pays off even when experienced. Here are Elm's versions for comparison:

  > myfun 1 'c' 3

  -- TOO MANY ARGS ---------------------------------------------------------- REPL

  The `myfun` function expects 2 arguments, but it got 3 instead.

  5|   myfun 1 'c' 3
       ^^^^^

  Are there any missing commas? Or missing parentheses?

  > not (myfun 1)

  -- TYPE MISMATCH ---------------------------------------------------------- REPL

  The 1st argument to `not` is not what I expect:

  5|    not (myfun 1)
             ^^^^^^^

  This `myfun` call produces:

      a -> Bool

  But `not` needs the 1st argument to be:

      Bool

Besides being 'beginner friendly', this just takes away cognitive overhead when you can glance at an error message and immediately know what happened.

(Edit: Formatting. Note that in the console output the arrows actually line up with the source of the errors)

It doesn't look so bad to me. It's telling you that a list type was expected but you passed it a function that returns a list instead.

You're right though that OCaml isn't the kind of language you can pick up in an afternoon.

Tiny quibble that I wouldn’t normally post but I think illustrates the readability issues here:

Should that be “Result set type was expected”, not “list”?

No, I think this: Result_set.t list

means a list whose items are of type Result_set.t.

Also I think the notation X.t is idiomatic for a type defined by module X.

Regardless the function does not return a list.

It looks like I missed a parenthesis.

Can you explain (rather than state) what readability issue you think exists here?

I don't know if you see a real readability issue or not, but I do think it's easy to jump to "hard to read" when that's only true in a certain parochial sense: it's not what we're used to.

> you passed it a function that returns a list instead

I don't know OCaml really, but I would read that intuitively as a list of (functions that return Result_set.t), which is not compatible with a list of Result_set.t.

This seems exactly the same to me just with more complicated type names.

Indeed it is :)

Dropping an example in TryOcaml[0]:

    let f : int -> int = fun _ -> 1 ;;
    val f : int -> int = <fun>
    let ex : int list = [f] ;;
    Line 1, characters 21-22:
    Error: This expression has type int -> int but an expression was expected of type
      int

[0]: https://try.ocamlpro.com/

Stockholm syndrome arrives quite quick in this case, as you become accustomed to this form as meaning "you're missing an arg"

I agree the first 6 months of this is mind-numbingly frustrating.

This is not Stockholm syndrome. It's just learning a pattern.

That's one of the top few specialties tasks for humans. After you learn it, you immediately know its meaning, it becomes easy with no thinking at all required.

Anyway, the plain English example from Elm on the sibling comment is much better, because it's already easy to understand before you learn to recognize it.

What does Stockholm syndrome even have to do with this?

> Stockholm syndrome

Is a lie fabricated to deflect criticism of the person who coined the term via abusive ad hominem of the critic.

Surprisingly, that tends to predict the quality of arguments invoking Stockholm syndrome pretty well, too.

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