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2020-10-02 15:44:42 <Guest18> safeDiv :: (Num a, Fractional a, Eq a) => a -> a -> Either String a
2020-10-02 15:44:43 <Guest18> safeDiv _ 0 = Left "The divider is zero, computation failed"
2020-10-02 15:44:43 <Guest18> safeDiv x y = Right $ x/y
2020-10-02 15:44:56 <Guest18> Ah, forgot to get rid of $
2020-10-02 15:44:59 × machinedgod quits (~machinedg@d67-193-126-196.home3.cgocable.net) (Ping timeout: 240 seconds)
2020-10-02 15:45:03 <Guest18> Anyway, you mean like this?
2020-10-02 15:45:08 <ski> Guest18 : "Is this the way to do it?" -- anyway, it also depends on how you intend to use it, e.g. what other possibly-failing operation could you want to use, together with this ? or in what ways (if any) would you want to handle/catch the failure ?
2020-10-02 15:45:13 <ski> yes
2020-10-02 15:45:48 <ski> although `Num a' is redundant, since `Num' is a superclass of `Fractional'
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2020-10-02 15:46:32 <Guest18> Well, I know this is a bad example, because it can only fail in one way. But say for some reason, I hate dividing by 3, so i add a pattern for that and return another string or something to signify that
2020-10-02 15:46:48 <Guest18> So i can see what actually happened
2020-10-02 15:46:59 <Guest18> Is this how Either is supposed to be used?
2020-10-02 15:47:32 <ski> (i've occasionally used e.g. `Either Integer' as a monad, or idiom. aborting some operation involving "attempts", reporting the first `Integer' that failed an attempt)
2020-10-02 15:48:22 <ski> Guest18 : it's really hard to say, without seeing the context in which you intend to use this
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2020-10-02 15:49:37 <cpressey> Guest18: Yes, Either can be used this way.
2020-10-02 15:49:49 <ski> if you just want to represent possible failure, and don't care about a situation where you may want to distinguish between the possible failures of multiple operations, i'd just use `Maybe'
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2020-10-02 15:50:27 <ski> if you do want to be able to distinguish between failures from different operations, then it could make more sense to use `Either'
2020-10-02 15:50:41 <Guest18> Ok, then, once i get the result, how do i extract the value? Through pattern-matching?
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2020-10-02 15:51:57 <Uniaika> Guest18: you may, but helper functions such as `either` exist as well, to inline the case analysis
2020-10-02 15:52:03 <ski> Guest18 : again, "it depends"
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2020-10-02 15:53:44 <ski> if this is a "one off" case, sure, you can do that. or things like `maybe',`fromMaybe',`either'. or sometimes things like `catMaybe',`mapMaybe',`partitionEithers' could be what you're looking for
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2020-10-02 15:54:31 <ski> but often you e.g. want to "short-circuit" a combination of multiple possibly failing operations, so that if one operation fails, the "next one" won't be tried, instead failing the whole combination
2020-10-02 15:54:51 <ski> Guest18 : this is one thing that `Monad' (and `Applicative') is useful for
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2020-10-02 15:59:51 <ski> @let safeDiv :: (Integral a,Eq a) => a -> a -> Maybe a; safeDiv _ 0 = Nothing; safeDiv n d = Just (div n d)
2020-10-02 15:59:53 <lambdabot> Defined.
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2020-10-02 16:00:02 <monochrom> It always comes down to pattern matching. The only question is do you handwrite every time or do you refactor and reuse.
2020-10-02 16:00:04 <ski> > maybe "Oops !" show (for [1,2,3,4,5,6] (\d -> safeDiv 12 d))
2020-10-02 16:00:08 <lambdabot> "[12,6,4,3,2,2]"
2020-10-02 16:00:11 <ski> > maybe "Oops !" show (for [1,2,3,0,5,6] (\d -> safeDiv 12 d))
2020-10-02 16:00:14 <lambdabot> "Oops !"
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2020-10-02 16:01:00 <ski> Guest18 : that's an example of performing a sequence of possibly-failing operations, short-circuiting so that it fails (doesn't continue) as soon as an individual operation fails
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2020-10-02 16:06:52 <Guest18> ski: I see, thank you for the thorough explanation. You too, Uniaika
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2020-10-02 16:08:49 niko is now known as ping
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2020-10-02 16:11:07 <ski> > (either id absurd . (`evalStateT` 1) . forever) (do n <- get; when (n >= 1000) (throwError n); put (2 * n)) -- Guest18, another example, doubling (starting from `1'), aborting when we reach at least a thousand
2020-10-02 16:11:11 <lambdabot> 1024
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2020-10-02 16:12:39 <ski> (if we didn't abort with `throwError' (which corresponds to `Nothing' or `Left' above, expressing (short-circuiting) failure), this would go on `forever')
2020-10-02 16:15:10 <ski> sometimes, one might also have an alternative branch (or a bunch of them), to try, in case the "main computation branch" failed. then one could reach for library operations having to do with `MonadPlus' (or `Alternative')
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2020-10-02 16:16:46 <Guest18> Thank you, I'm gonna go over your examples later, i need to go now
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2020-10-02 16:20:13 <fresheyeball> anyone know how to get haddock themes to work?
2020-10-02 16:24:58 hackage blank-canvas 0.7.3 - HTML5 Canvas Graphics Library https://hackage.haskell.org/package/blank-canvas-0.7.3 (ryanglscott)
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