Improving readability of type/casting error messages

[cowwoc]

Hi,

With the addition of lambdas in Java8, I'm running into generics error
messages more frequently than ever before and the messages have gotten less
and less readable over time.

I'll start by discussing a seemingly benign error message:

incompatible types: bad return type in lambda expression
    Promise<Optional<GetPostalCode>> cannot be converted to Optional

for this code:

		Promise<Optional<GetPostalCode>> result = current.map(child ->
		{
			// do something
			return Optional.empty();
		}).orElseGet(() ->
		{
			return getPostalCode();
		});
		return result;

The line triggering the error is: "return getPostalCode()"

Here's my problem:

1. getPostalCode() returns "Promise<Optional<GetPostalCode>>"
2. "result" is of the same type.
3. Looking at this code, it's not obvious why the compiler is trying to cast
the output to "Optional"

I've run across this kind of problem very often. The problem I am referring
to is not the specific error message but rather the fact that you're forcing
developers to reverse engineer the compiler in their head to figure out what
is going wrong.

Instead of forcing developers to do figure out why the compiler is
attempting a seemingly wrong cast, couldn't the error message explain it
explicitly? I don't mind passing extra command-line parameters into the
compiler to get more verbose explanations, so long as this is possible.

The second kind of error I am running into is:

method thenApply in class Promise<T> cannot be applied to given types;
  required: Function<? super PostProperty,? extends U>
  found: this::addProperty
  reason: cannot infer type-variable(s) U
    (argument mismatch; bad return type in method reference
      void cannot be converted to U)
  where U,T are type-variables:
    U extends Object declared in method <U>thenApply(Function<? super T,?
extends U>)
    T extends Object declared in class Promise

I don't know about you, but (as a human being) I find this message very hard
to read. It sounds as if the message was written for computers to parse, not
people. What the compiler is actually trying to say is that
"this::addProperty returns void but thenApply() expects it to return a type
that extends Object".

Is it possible for someone to investigate improving these messages?

Thank you,
Gili



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