Anthony Clayden
Browsing some docos for a completely other purpose, I came across this code:
f' [x, y] | True <- x, True <- y = True f' _ = False
(In User Guide 6.7.4.5 Matching of Pattern Synonyms.)
That business with the comma and left-arrows? They're 'Pattern guards', Language Report 2010 section 3.13. That also specs 'local bindings' introduced by `let`.
In 10 years of reading Haskell code, I've never seen them. Does anybody use them? Are they more ergonomic than guards as plain Boolean expressions? Are 'local bindings' any different vs shunting the `let` to the rhs of the `=`?
I'd write that code as:
f'' [x@True, y@True] = True f'' _ = False
I can see the rhs of the matching arrow could in general be a more complex expression. But to express that you could put a more complex Boolean guard(?)
I've used it occasionally, e.g when dealing with exceptions. I think this is the most recent example: ``` lastExceptionHandler :: LoggerSet -> SomeException -> IO () lastExceptionHandler logger e | Just TimeoutThread <- fromException e = return () | otherwise = do logFatalIoS logger $ pack $ "(ws) uncaught exception: " <> displayException e flushLogStr logger ``` (That's the exception handler I installed with `setUncaughtExceptionHandler` in a web service to deal with https://github.com/yesodweb/wai/issues/852) /M -- Magnus Therning OpenPGP: 0x927912051716CE39 email: magnus@therning.org @magthe@mastodon.technology http://magnus.therning.org/ Action is the foundational key to all success. — Pablo Picasso