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Logs: liberachat/#haskell

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2021-06-26 16:48:23 → peterhil joins (~peterhil@dsl-hkibng32-54f849-252.dhcp.inet.fi)
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2021-06-26 16:57:56 <dminuoso> TheCommieDuck: If this was recurring, you could write generic code to do this for you.
2021-06-26 16:58:05 <dminuoso> Or perhaps better yet TH code
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2021-06-26 17:13:18 <TheCommieDuck> I've gotten FooStore done in TH
2021-06-26 17:13:32 <TheCommieDuck> but it still feels like someone's probably done it properly before me
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2021-06-26 17:17:05 <dminuoso> Mmm, this usage seems exotic to me.
2021-06-26 17:17:40 → alex3 joins (~alex3@BSN-77-82-41.static.siol.net)
2021-06-26 17:18:51 <dminuoso> TheCommieDuck: How did you implement it? Turn each IntMap into a list of pairs, and then scan all intmaps in parallel?
2021-06-26 17:19:02 <dminuoso> (or rather, scan all lists)
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2021-06-26 17:20:48 <TheCommieDuck> the TH generated has each intMap field as typenameStore, and when defining a new Foo I can make a lens with functors and intmap.union
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2021-06-26 17:25:48 <motte_> hi, i'm having trouble writing a function that takes a string and a type and checks whether the string can be parsed to that type. this is what i've got so far: https://paste.tomsmeding.com/cBYkcZkH
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2021-06-26 17:27:13 <int-e> there's no types at runtime, so you can't pattern match on types. you can use Data.Typeable to have an explicit runtime type representation, but usually there's a better way...
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2021-06-26 17:27:34 <dminuoso> motte_: heh, I think what you are looking for already is readMaybe
2021-06-26 17:27:38 <dminuoso> % :t readMaybe
2021-06-26 17:27:39 <yahb> dminuoso: ; <interactive>:1:1: error: Variable not in scope: readMaybe
2021-06-26 17:27:46 × talismanick quits (~user@2601:644:8502:d700::f19d) (Ping timeout: 256 seconds)
2021-06-26 17:27:49 <dminuoso> % import Text.Read (readMaybe)
2021-06-26 17:27:50 <yahb> dminuoso:
2021-06-26 17:27:51 <dminuoso> % :t readMaybe
2021-06-26 17:27:51 <yahb> dminuoso: forall {a}. Read a => String -> Maybe a
2021-06-26 17:28:43 <motte_> well, i have the possible types in a sum type (data Type in the example)
2021-06-26 17:28:57 <dminuoso> motte_: We can think of both the type variable `a` and the dictionary `Read a` as being term-level arguments to the function. In GHC Haskell this is really the case, except GHC will automatically fill in those arguments using inference.
2021-06-26 17:29:22 → alx741 joins (~alx741@181.196.69.41)
2021-06-26 17:29:22 <dminuoso> So:
2021-06-26 17:29:27 <dminuoso> % :t isJust . readMaybe
2021-06-26 17:29:27 <yahb> dminuoso: ; <interactive>:1:10: error:; * Ambiguous type variable `a0' arising from a use of `readMaybe'; prevents the constraint `(Read a0)' from being solved.; Probable fix: use a type annotation to specify what `a0' should be.; These potential instances exist:; instance forall a. Read a => Read (ZipList a) -- Defined in `Control.Applicative'; instance forall a. Read a => Re
2021-06-26 17:29:28 <int-e> ah, sorry, I missed the Type type
2021-06-26 17:29:35 <int-e> or rather, its significance
2021-06-26 17:29:50 <dmj`> motte_: it can work if you use Proxy as a witness to the type at runtime
2021-06-26 17:30:01 <dminuoso> % :t canParse :: Read a => (Proxy a) -> String -> Bool; canParse _ = isJust . readMaybe
2021-06-26 17:30:01 <yahb> dminuoso: ; <interactive>:1:50: error: parse error on input `;'
2021-06-26 17:30:04 <dminuoso> dmj`: Ah you beat me
2021-06-26 17:30:15 <dminuoso> % canParse :: Read a => (Proxy a) -> String -> Bool; canParse _ = isJust . readMaybe
2021-06-26 17:30:15 <yahb> dminuoso: ; <interactive>:104:74: error:; * Could not deduce (Read a0) arising from a use of `readMaybe'; from the context: Read a; bound by the type signature for:; canParse :: forall a. Read a => Proxy a -> String -> Bool; at <interactive>:104:1-49; The type variable `a0' is ambiguous; These potential instances exist:; instance forall a. Read a => Re
2021-06-26 17:30:19 <int-e> motte_: but there's nothing connecting the data constructor named Int to the type named Int, you have to make the connection yourself
2021-06-26 17:30:25 <dmj`> dminuoso: ¯\_(ツ)_/¯
2021-06-26 17:30:38 <dminuoso> % canParse :: forall a. Read a => (Proxy a) -> String -> Bool; canParse _ = isJust . readMaybe @a
2021-06-26 17:30:39 <yahb> dminuoso:
2021-06-26 17:30:41 <dminuoso> motte_: ^-
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2021-06-26 17:35:13 <motte_> hmm, which extensions does this need?
2021-06-26 17:35:27 <motte_> RankNTypes, TypeApplications, what else?
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2021-06-26 17:36:46 <geekosaur> ScopedTypeVariables?
2021-06-26 17:38:21 <motte_> ah, yes, got it
2021-06-26 17:39:08 <dmj`> motte_: no rank n types needed
2021-06-26 17:39:15 <dmj`> motte_: just STV and TA
2021-06-26 17:39:22 <dminuoso> motte_: You can avoid the usage of TypeApplications with a type ascriptions instead. You just need ScopedTypevariables
2021-06-26 17:40:35 <dminuoso> % canParse :: forall proxy a. Read a => proxy a -> String -> Bool; canParse _ = isJust . (readMaybe :: String -> Maybe a) -- motte_
2021-06-26 17:40:35 <yahb> dminuoso:
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2021-06-26 17:44:55 <motte_> so using this function would like: canParse (Proxy :: Proxy Int) "5"
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2021-06-26 17:49:25 <motte_> what if the Int is a member of a sum type and also comes from a variable?
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2021-06-26 17:53:16 <dsal> What is the use case for `canParse`? Normally you'd just parse and if you can't, you'd know because it didn't work. If you can, you'd have the value you get from parsing.
2021-06-26 17:53:28 <dmj`> motte_: you'll probably need an instance of Read
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2021-06-26 17:58:01 <euandreh> I see Real World Haskell has a chapter on parsec. Are there other books that cover it?
2021-06-26 17:58:11 <euandreh> (other than online articles, blog posts, etc.)
2021-06-26 18:00:35 <dsal> The biggest bit of the learning curve I found for *parsec is that it's not actually that complicated and barely does anything at all... but it seems like it should be complicated and do a lot, so I also got frustrated when I couldn't find great material that explained it all.
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2021-06-26 18:00:55 <dsal> Then I just started reading and writing parsers.
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