Haskell Kwargs Dream Syntax

kwargs.hs

-- dream syntax for Haskell kwargs
mkString :: Show a =>
    { pfx :: String, ifx :: String, sfx :: String } -> [a] -> String
mkString { pfx, ifx, sfx } xs =
    pfx ++ infixed xs ++ sfx
    where
    infixed [] = ""
    infixed [x] = show x
    infixed (x:xs) = prepend x (infixed xs)
    prepend x s = show x ++ ifx ++ s


-- you can call `mkString` using record syntax
bracesAndSemicolon :: Show a => [a] -> String
bracesAndSemicolon = mkString { pfx = "{", ifx = ";", sfx = "}" }


-- you can call `mkString` using positional syntax
anglesAndColon :: Show a => [a] -> String
anglesAndColon = mkString "<" ":" ">"


-- you can partially apply the record
parensAndYouPickIfx :: Show a => { ifx :: String } -> [a] -> String
parensAndYouPickIfx = mkString { pfx = "(", sfx = ")" }

-- syntax is purely sugar. core representation of mkString is
--
--     Show a -> String -> String -> String -> [a] -> String
--
-- as such, `{ pfx :: String, ifx :: String, sfx :: String }` is
-- not a type, so no modifications to the type system are needed,
-- only modifications to headers (.hi files) and desugaring step.
-- also along those lines, there are no selectors, you must mach
-- out the named arguments in your function binder (or use
-- positional binding).
--
-- this provides a lightweight alternative to the below patterns:
--
--     type Pfx = String
--     type Ifx  = String
--     type Sfx = String
--
--     mkString :: Show a -> Pfx -> Ifx -> Sfx -> [a] -> String
--
-- which introduces extra type-level noise without much dummy-proofing
--
-- and
--
--     newtype Pfx = Pfx String
--     newtype Ifx  = Ifx  String
--     newtype Sfx = Sfx String
--
--     mkString :: Show a -> Pfx -> Ifx -> Sfx -> [a] -> String
--
-- which provides great dummy-proofing but also adds type-level noise
-- and is kind of onerous to use.