Hmm, I done it with: infixl 9 ||| data a ||| b = A a|B b deriving Show instance (Read a, Read b) => Read (a ||| b) where readPrec = parens $ do a <- (A <$> readPrec) <|> (B <$> readPrec) return a so parse looks like: read "1" :: Int ||| Char ||| String but I'm not sure is other more classical way to do it...
Hello, All!
I have types A, B, C... They have instances of class Read, Show, SomethingElse, ... I want to keep them as one collection. For example, I can use such datatype for it:
data Tags = forall a. Read a => Tags [a]
Now I want to parse "some-string" into A or B or C or ... but without to create type's sum of A|B|C|...! All those types have `Read` instance but Haskell will not find what instance to use, right? How to parse string with alternatives where each alternative returns `ReadP A` or `ReadP B`, etc? My intuition is that I should make `x <|> y <|> z <|> ...` where x,y,z - parses string into A, B, C, ... so they are expressions like `readPrec :: ReadPrec A`, etc. But how to pass types into such expression? I must to make expression folding (with `<|>`) `readPrec::someType`, where `someType` is item of types list `{A, B, C, ...}`
May be it's possible to be done with HList-like types, like: HSet, HVect or similar, but I don't know:
- how to iterate/fold over types - I'm not sure that HSet/HVect keeps types (like `A ::: B ::: C ::: ...`)
My idea is to have some function which can parse string into one of A|B|C|... with call like:
myParse "some-string" :: (Something A ::: B ::: C ::: D)
(suppose `:::` is type-level "cons"). So, `myParse` is polymorphic and it can fold/iterate over type-list items (to call appropriate `readPrec`).
Is it even possible?
In languages with reflection I can pass list of types (type - is usual value) and call type's methods, but how to solve similar in Haskell?
=== Best regards, Paul