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David pushed to branch wip/or-pats at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
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Commits:
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<strong style="font-weight: bold;"><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/4c070b086e8bc7c79117ee3764dc0ae13ba2fa95">4c070b08</a></strong>
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<span> by David Knothe </span> <i> at 2023-03-03T13:10:28+01:00 </i>
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<pre class="commit-message" style='white-space: pre-wrap; display: block; font-size: 0.875rem; color: #333238; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: #fbfafd; border-radius: 2px; margin: 0; padding: 8px 12px; border: 1px solid #dcdcde;'>Implement Or Patterns (Proposal 0522)
This commit introduces a language extension, `OrPatterns`, as described in proposal 0522.
It extends the syntax by the production
`pat -> (one of pat1, ..., patk)`.
The or-pattern `pat` succeeds iff one of the patterns `pat1`, ..., `patk` succeed, in this order.
Currently, or-patterns cannot bind variables. They are still of great use as they discourage the use of wildcard patterns in favour of writing out all "default" cases explicitly:
```
isIrrefutableHsPat pat = case pat of
...
(one of WildPat{}, VarPat{}, LazyPat{})
= True
(one of PArrPat{}, ConPatIn{}, LitPat{}, NPat{}, NPlusKPat{}, ListPat{})
= False
```
This makes code safer where data types are extended now and then - just like GHC's `Pat` in the example when adding the new `OrPat` constructor. This would be catched by `-fwarn-incomplete-patterns`, but not when a wildcard pattern was used.
- Update submodule haddock.
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docs/users_guide/exts/or_patterns.rst
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