Indicate expected versus actual types in type errors

[i-am-the-slime]

I never know what the compiler wants from me when it says:

Could not match type A with type B

Which one does it expect? Which one is the one I supply?
For people like me it's not always obvious which type I am supplying.
I don't really see any downside to changing this to:

Could not match expected type A with actual type B

I don't know about any tooling that might depend on this exact message.

[i-am-the-slime]

I think it could be even nicer if the compiler could insert a little (?helpme (thingOfTheWrongType)) and suggest possible fixes but that's most probably out of scope.

[garyb]

We've had issues about this previously, and did attempt to change the error to work this way... but unfortunately the type checker isn't able to know for sure whether A or B is the type that is actually desired.

[hdgarrood]

Yeah, I’m certain there is already an issue for exactly this, possibly even open.

[i-am-the-slime]

I see. Sorry guys!

[i-am-the-slime]

Actually maybe we could make that explicit in the message? I remember trying to find the order by making some minimal example. Since there is no order this could be misleading. I know there is nothing in the message suggests that there is an order but still. The message could have a little "in no particular order" disclaimer.

[hdgarrood]

Previously: #3111 #1577 #1583

[hdgarrood]

Actually sorry, I take back what I said before: while we do have discussion which notes that identifying expected vs actual types would be useful, there doesn’t appear to be an open issue for actually doing so.

[hdgarrood]

So we might want to reopen this perhaps?

[JordanMartinez]

I think this should be reopened. At the very least, it would track that the issue does exist.

[ajnsit]

I wrote this on discord, but thought I would add it as a suggestion here as well -

Looking at the error message in this issue #1577 - Some very useful information is partly present but buried - "checking that expression 4 has type String". Is it possible to put that information up front?

For example the error messages could say something like -

Error occurred while checking that
expression
    4
  has type
    String

Could not match type
    Int
  with type
    String
while trying to match type Int
  with type String

in value declaration foo