Import suggestion for missing newtype constructor, all types constructor and indirect overloadedrecorddot fields #4516
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In a context where we want to `coerce` to a type for which constructor
is not in scope, GHC fails with (e.g., for `Sum Int`):
```
• Couldn't match representation of type ‘Int’ with that of ‘Sum Int’
arising from a use of ‘coerce’
The data constructor ‘base-4.18.2.1:Data.Semigroup.Internal.Sum’
of newtype ‘Sum’ is not in scope
```
This code action detects the missing `newtype` and suggests to add the
required import.
This is convenient because otherwise the user need to interpret the
error message and most of the time manually find which module and type to import.
Note that a better implementation could try to decet that the type is
already imported (if that's the case) and just suggest to add the
constructor (e.g. `(..)`) in the import list, but this is too much
complexity to implement. It could lead to duplicated import lines which
will be "cleaned" by formatter or other extensions.
For example, `deriving instance Generic (Sum Int)`, but it should work
for other deriving which indirectly requires a complete access to the
type constructor.
```
Can't make a derived instance of ‘Generic (Sum Int)’:
The data constructors of ‘Sum’ are not all in scope
so you cannot derive an instance for it
```
For example, the following code `foo.titi` when the type of `foo` (e.g.
`Bar` here is not in scope and not from an already imported module (e.g.
the type exists indirectly because here `foo :: Bar` comes from another module).
If the module which contains `Bar` is already imported, GHC already
gives an hint to add `titi` to the `import Bar` line and this is already
correctly handled by HLS.
```
No instance for ‘HasField "titi" Bar.Bar String’
arising from selecting the field ‘titi’
```
This correct previous commit by handling ghc 9.4 parethensis instead of "tick".
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|
Rebased and fixed test for GHC 9.4. |
soulomoon
reviewed
Mar 19, 2025
| -- "foo" (Bar Int)...`, e.g. see the parenthesis | ||
| | Just [_module, name] <- matchRegexUnifySpaces x "No instance for [‘(].*HasField \"[^\"]+\" ([^ (.]+\\.)*([^ (.]+).*[’)]" | ||
| = Just $ NotInScopeThing name | ||
| | Just [_module, name] <- matchRegexUnifySpaces x "No instance for [‘(].*HasField \"[^\"]+\" \\(([^ .]+\\.)*([^ .]+)[^)]*\\).*[’)]" |
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This seems complex.
perhapes more unit test on these patterns. Thanks
I mean just plain unit test for matchRegexUnifySpaces x "No instance for [‘(].*HasField \"[^\"]+\" ([^ (.]+\\.)*([^ (.]+).*[’)]" and matchRegexUnifySpaces x "No instance for [‘(].*HasField \"[^\"]+\" \\(([^ .]+\\.)*([^ .]+)[^)]*\\).*[’)]"
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This MR contains 3 commits which improves the import suggestion code actions:
newtypeconstructor is not in scope. This usually happen whencoerceingnewtype.deriving instance Generic BarwhenBar(the type) may be in scope, but not the construtors (e.g. requiringBar(..)).OverloadedRecordDotcontext, such asfoo.barwherefoo :: FooandFoo(bar)is not in scope. Note that this is already partially handled by HLS in the case where the module containingFoois already imported, but not the fields. In this context, GHC is kind enough to give us an hint (something like "maybe add fieldbarto import line...") and this is already handled by HLS. However, if the module containingFoois not already imported, it does not work.All of these case usually happen when types are "indirectly" in scope. For example, consider the followings:
In the module
Usage, the typeFoois not explicitly imported, it exists just indirectly becausefoois imported fromValue.Discussion
I tried to take care of multiples patterns in the regex used to match the error messages, such as qualified or not qualified types or class, as well as polymorphic or not. If you see a case not taken into account, please tell me.
Tests
I've added simple tests for the
coerceandinstance Generic"cases" and a more involved test for the overloaded record dot cases.I've developed this using GHC 9.10. I'll test on other version of GHC asap, perhaps some adaptations of the regex need to be introduced.