Johnni Winther
29c5b5da9a
This adds support for exhaustiveness checking of list types by
contextually dividing the list type into relevant cases for checking.
For instance, the exhaustiveness can be achieved by a single pattern
case [...]:
or by two disjoint patterns:
case []:
case [_, ...]:
When checking for exhaustiveness, witness candidates are created and tested against the available cases. This means that the chosen candidates must be matched by at least one case or the candidate is considered a witness of non-exhaustiveness.
Looking at the first example, we could choose `[...]`, the list of
arbitrary size, as a candidate. This works for the first example, since the case `[...]` matches the list of arbitrary size. But if we tried to use this on the second example it would fail, since neither `[]` nor `[_, ...]` fully matches the list of arbitrary size.
A solution could be to choose candidates `[]` and `[_, ...]`, the empty list and the list of 1 or more elements. This would work for the first example, since `[...]` matches both the empty list and the list of 1 or more elements. It also works for the second example, since `[]` matches the empty list and `[_, ...]` matches the list of 1 or more elements.
But now comes a third way of exhaustively matching a list:
case []:
case [_]:
case [_, _, ...]:
and our candidates no longer work, since while `[]` does match the empty
list, neither `[_]` nor `[_, _, ...]` matches the list of 1 or more
elements.
This shows us that there can be no fixed set of witness candidates that we can use to match a list type.
What we do instead, is to create the set of witness candidates based on the cases that should match it. We find the maximal number, n, of fixed, i.e. non-rest, elements in the cases, and then create the lists of sizes 0 to n-1 and the list of n or more elements as the witness candidates.
Change-Id: I27d594699a65510636a2b7811c60c261a02f9b57
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/287680
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>
FE/analyzer shared code
This package contains logic that is shared between the front_end and analyzer packages. It is intended solely to facilitate development of the Dart SDK, and is not intended for use by end users. In particular, this package has no public API, so no guarantee is made of compatibility between one version of the package and the next.
End users should consider using the analyzer package to analyze Dart source code.