[Haskell-cafe] Pattern guards seen in the wild?

[Magnus Therning]

Anthony Clayden <anthony.d.clayden at gmail.com> writes:

> 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 at True, y at 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 at therning.org
@magthe at mastodon.technology       http://magnus.therning.org/

Action is the foundational key to all success.
     — Pablo Picasso
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