This reverts commit 6e4fd5f06a1ae6138659b0073b4e2b375a499588.
This idea didn't work out because cloning the type and storing it on a
variable still resulted in the solver trying to uify the variable with
the type. When there were errors, which there certainly would be if we
tried to unify the variable with a structure that had nested flex/rigid
vars, the nested flex/rigid vars would inherit those errors, and the
program wouldn't typecheck.
Since the motivation here was to expose the signature type to
`reporting` so that we could modify it with suggestions, we should
instead pass that information along in something analogous to the
`Expected` struct.
There is still a potential for conflicts here, because we don't look at
type variables introduced _prior_ to this annotation. However, this
should be okay in most cases.
To provide better error messages and suggestions related to changing
type annotations, we now pass annotation type signatures all the way
down through the constraint solver. At constraint generation we
associate the type signature with a unique variable, and during error
reporting, we pull out an `ErrorType` corresponding to the original type
signature, by looking up the unique variable. This gives us two nice
things:
1. It means we don't have to pass the original, AST-like type
annotation, which can be quite large, to everyone who looks at an
expectation.
2. It gives us a translation from a `Type` to an `ErrorType` for free
using the existing translation procedure in `roc_types::subs`,
without having to create a new translation function.
This makes it easier for error reporting to find the relevant
annotations that were part of a type error, and display that in the
error message presented to a user.
Previously, a program like
```roc
word = "word"
if True then 1 else "\(word) is a word"
```
would report an error like
```
── TYPE MISMATCH ───────────────────────────────────────────────────────────────
This `if` has an `else` branch with a different type from its `then` branch:
3│ if True then 1 else "\(word) is a word"
^^^^^^^^^^^^^^^^^^
This concat all produces:
Str
but the `then` branch has the type:
Num a
I need all branches in an `if` to have the same type!
```
but this is a little bit confusing, since the user shouldn't have to
know (or care) that string interpolations are equivalent to
concatenations under the current implementation.
Indeed we should make this fully transparent. We now word the error
message by taking into account the way calls are made. To support the
case shown above, we introduce the `CalledVia::Sugar` variant to
represent the fact that some calls may be the result of desugaring the
surface syntax.
This commit also demonstrates the usage of `CalledVia` to produce better
error messages where we use binary comparison operators like `<`. There
are more improvements we can make here for all `CalledVia` variants, but
this is a good starting point to demonstrate the usage of the new
procedure.
Closes#1714
