[thinkpad20]

Many of the error messages become clearer with experience, but this is mostly due to developing an intuition for what is actually wrong with your code based on the error message you see, not due to the helpfulness of the error message. Often the actual source of the error is in a different place than what is reported. Consider what happens if you mean to concatenate two strings but forget to put `++` in there:

    Prelude> "foo" "bar"

    <interactive>:3:1:
        Couldn't match expected type ‘[Char] -> t’
                    with actual type ‘[Char]’
        Relevant bindings include it :: t (bound at <interactive>:3:1)
        The function ‘"foo"’ is applied to one argument,
        but its type ‘[Char]’ has none
        In the expression: "foo" "bar"
        In an equation for ‘it’: it = "foo" "bar"

Yikes. Even worse, consider a similar situation with numbers:

    Prelude> 1 2

    <interactive>:2:1:
        Non type-variable argument in the constraint: Num (a -> t)
        (Use FlexibleContexts to permit this)
        When checking that ‘it’ has the inferred type
          it :: forall a t. (Num a, Num (a -> t)) => t

Also the parser errors in Haskell are terrible. That the community has so long put up with "parse error (possibly incorrect indentation or mismatched brackets)" is a marvel to me, and is one of the most irritating errors to fix because of the (at least apparent) simplicity of improving it.

[ob]

And that is exactly why people love Elm:

  > "foo" "bar"
  -- TYPE MISMATCH --------------------------------------------- repl-temp-000.elm
  
  You are giving an argument to something that is not a function!
  
  3|   "foo" "bar"
             ^^^^^
  Maybe you forgot some parentheses? Or a comma?

Also

  > "foo" + "bar"
  -- TYPE MISMATCH --------------------------------------------- repl-temp-000.elm
  
  The left argument of (+) is causing a type mismatch.
  
  3|   "foo" + "bar"
       ^^^^^
  (+) is expecting the left argument to be a:
  
      number
  
  But the left argument is:
  
      String
  
  Hint: To append strings in Elm, you need to use the (++) operator, not (+).
  <http://package.elm-lang.org/packages/elm-lang/core/latest/Basics#++>

[the_af]

Interesting! Thanks for the example.

How do compiler errors look in Elm with generic functions involved, about which the compiler doesn't have any helpful hardcoded knowledge? (I.e. no "+" or strings involved).

[elros]

If you mean typeclasses, Elm doesn't have them.

Regarding hardcoded messages, there's been at some point talk in the community regarding allowing library authors to have some sort of hook to compiler that would make it possible to customize specific error messages along with a package.

[HelloNurse]

The first error message is good: it helpfully includes the exact expression that craps out, and it means, matching intuition about how Haskell should be compiled, that the expression "foo" "bar" could only make sense if "foo" were a function from [Char] (i.e. the type of "bar") to some type t, but unfortunately "foo" is a [Char]: everyone should understand it easily, even if gratuitously introducing other names (it and its type t) and details about the compiler making up type equations is highly inelegant.

The second error message, on the other hand, proves that the first one is a lucky accident.

[AlexCoventry]

Everything you say is true, but it took me about 90s of staring at the Haskell error before I parsed out exactly what it was objecting to, even though I knew exactly how the code was broken. Whereas it took me no discernible time to understand the Elm error messages. Some of it could be that parsing the Haskell message primed me for the Elm message, but it doesn't seem that way to me.

[MustardTiger]

>The second error message, on the other hand, proves that the first one is a lucky accident.

No, the second error message just shows that there's more options there so the error message is vague and not very helpful. The first one is not an accident, it is the standard type mismatch error.

[HelloNurse]

My point is that 1 2 should give the same type of error message as "foo" "bar", because it is the same kind of error (1 isn't a function with one or more arguments).

The compiler exposes that when numbers are involved it attempts (and fails) some fancy and unexpected reasoning involving forall, while with plain type constructors like [Char] it attempts (and fails) straightforward and intelligible pattern matching.

Figuring out the type of 1 and 2 might be more difficult than figuring out the type of "foo" and "bar", but the special rules should be hidden: a compiler that cares about practical usage would provide unified and clear error messages, possibly listing sets of alternative types it was unable to choose from.

[MustardTiger]

>My point is that 1 2 should give the same type of error message as "foo" "bar", because it is the same kind of error

And my point is that no it should not. "1 2" is more ambiguous, it could be more things, hence there is a more ambiguous error message.

[the_af]

Not saying many Haskell compiler errors are obscure, especially for the uninitiated, but the particular error you picked as your first example is perfectly clear.

Haskell is telling you 'you tried to apply a string literal to an argument as if it were a function, but it's not! It's a string literal. By the way, the string I'm talking about is "foo", which you tried to apply to "bar" in the expression "foo bar"'.

Yikes? It seems pretty helpful to me :)

Your second example is better, though.

[thinkpad20]

Well like I said, once you see enough of these error messages you get a clear idea of how to parse them and understand what they're "really saying". But on first glance, and in particular for newcomers, the error message is rather arcane. And of course, in this instance it's quite straightforward because it's a very simple expression, with a rather clear problem. But consider when the error might be buried in a much more complicated expression, or the fact that it's not a function (or is a function, when you expected otherwise) might not be as clear. For example a curried function that has too many or not enough arguments applied.

Also, although from a compiler's point of view it might make sense, the wording "The function ‘"foo"’ is applied to one argument, but its type ‘[Char]’ has none" doesn't make sense. It's clearly not a function if it doesn't take an argument. The same error message would be far clearer if it said something similar to "The expression ‘"foo"’ is being used as a function, but it is of type ‘[Char]’".

Also these are just two examples I rattled off the top of my head; the fact is that while the compiler is incredibly helpful in making sure your code is correct, it's less helpful in explaining itself. But, I also realize that producing good error messages is hard, and (I assume) significantly less interesting to many on the GHC team.

[IanCal]

The output contains all you need, yes, but it would be nice if what you typed was added to the output. You wrote out a much more user friendly version.

Someone posted the output from Elm, which does this: https://news.ycombinator.com/item?id=11848467

Edit: checkout the elm error message page here http://elm-lang.org/blog/compilers-as-assistants

[the_af]

Agreed, Elm in these two examples is way nicer! It doesn't add information, but it presents it in a beginner-friendly way, which is commendable.

However, I'm left wondering... Elm seems to be using hardcoded knowledge about commonly used types such as strings and numbers in order to improve its error messages (I wonder why Haskell doesn't, by the way). What happens in Elm when an error occurs with more complex, user-defined types and operations?

[IanCal]

It looks like it's only the specific hint at the bottom that requires any custom knowledge about the types themselves.

If you're interested, there were a couple more examples shown elsewhere in this thread:

http://elm-lang.org/blog/compiler-errors-for-humans (an older version it seems)

https://twitter.com/st58/status/732908457217531904

https://twitter.com/GregorySchier/status/732830868562182144

Something I've just found is that the errors can also be output in a nice machine readable format, so the suggested fixes can be read in by your editor plugin.

And finally, this is wonderful, a specific repo for code that causes error messages and a request for people to submit error messages that are confusing: https://github.com/elm-lang/error-message-catalog

[masklinn]

In the "+" case, only the final hint is type-specific. The first error is kind-of type-specific but only distinguishes functions and non-functions.