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| author | Ryan Scott <ryan.gl.scott@gmail.com> | 2021-03-14 12:22:04 -0400 |
|---|---|---|
| committer | Marge Bot <ben+marge-bot@smart-cactus.org> | 2021-03-17 19:09:03 -0400 |
| commit | 26d26974f5d16a3b2c6ec84a739caab10c7d2b07 (patch) | |
| tree | e6bb784ac9b43ccf9da9b87a4886d76da8a6c6ff /libraries | |
| parent | 6b10163e0da90dfc0eeb52549598faa2c0527e37 (diff) | |
| download | haskell-26d26974f5d16a3b2c6ec84a739caab10c7d2b07.tar.gz | |
Document how GADT patterns are matched from left-to-right, outside-in
This adds some bullet points to the GHC User's Guide section on `GADTs` to
explain some subtleties in how GHC typechecks GADT patterns. In particular,
this adds examples of programs being rejected for matching on GADTs in a way
that does not mesh with GHC's left-to-right, outside-in order for checking
patterns, which can result in programs being rejected for seemingly
counterintuitive reasons. (See #12018 for examples of confusion that arose
from this.) In addition, now that we have visible type application in data
constructor patterns, I mention a possible workaround of using
`TypeApplications` to repair programs of this sort.
Resolves #12018.
Diffstat (limited to 'libraries')
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