[Haskell-cafe] Re: [Haskell] The initial view on typed sprintf and sscanf

[Bulat Ziganshin]

Hello Ryan,

Monday, September 1, 2008, 12:16:46 PM, you wrote:

of course this may be done with code generation tools (such as TH).
point of this research is to do this using type abilities of Haskell

Don, i think this should be impossible with IsString since the point
is that Haskell compiler should know types at compile time. IsString
can't convert "%d" into (X int) while converting "%s" into (X String)


> On Mon, Sep 1, 2008 at 1:00 AM, Don Stewart <dons at galois.com> wrote:
>> I also wonder if we could give a String syntax to the formatting
>> language, using -XOverloadedStrings and the IsString class.

> Probably easier with Template Haskell:

>> ghci -fth Sprintf.hs
*Sprintf>> :t $(sprintf "Hello %s, showing %S and %S")
> $(sprintf "Hello %s, showing %S and %S") :: (Show a1, Show a) =>
>                                             [Char] -> a -> a1 -> [Char]
*Sprintf>> $(sprintf "Hello %s, showing %S and %S") "Don" 1 (5,6)
> "Hello Don, showing 1 and (5,6)"

> Code follows, which could be ported to use the printf/scanf language
> Oleg defined.

>   -- ryan

> {-# LANGUAGE TemplateHaskell #-}
> module Sprintf where
> import Language.Haskell.TH

> data SprintfState =
>     SprintfState String (ExpQ -> ExpQ)

> flush :: SprintfState -> (ExpQ -> ExpQ)
> flush (SprintfState "" k) = k
> flush (SprintfState s  k) = (\e -> k [| $(litE $ StringL $ reverse s) ++ $e |])

> finish :: SprintfState -> ExpQ
> finish (SprintfState s k) = k (litE $ StringL $ reverse s)

addChar :: Char ->> SprintfState -> SprintfState
> addChar c (SprintfState s e) = SprintfState (c:s) e

addCode :: ExpQ ->> SprintfState -> SprintfState
> addCode k s = SprintfState "" (\e -> flush s $ [| $k ++ $e |])

> sprintf' :: SprintfState -> String -> ExpQ
> sprintf' k ('%':'S':cs) = [| \x -> $(sprintf' (addCode [| show x |] k) cs) |]
> sprintf' k ('%':'s':cs) = [| \s -> $(sprintf' (addCode [| s |] k) cs) |]
> sprintf' k ('%':'%':cs) = sprintf' (addChar '%' k) cs
> sprintf' k (c:cs)       = sprintf' (addChar c k) cs
> sprintf' k []           = finish k

sprintf :: String ->> ExpQ
> sprintf = sprintf' (SprintfState "" id)
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-- 
Best regards,
 Bulat                            mailto:Bulat.Ziganshin at gmail.com