Printf.idr

module Printf

%default total

data Format = FInt Format -- %d
            | FString Format -- %s
            | FOther Char Format -- [a-zA-Z0-9]
            | FEnd --

format : List Char -> Format
format ('%'::'d'::cs) = FInt (format cs)
format ('%'::'s'::cs) = FString (format cs)
format (c::cs) = FOther c (format cs)
format [] = FEnd

interpFormat : Format -> Type
interpFormat (FInt f) = Int -> interpFormat f
interpFormat (FString f) = String -> interpFormat f
interpFormat (FOther _ f) = interpFormat f
interpFormat FEnd = String

formatString : String -> Format
formatString s = format (unpack s)

toFunction : (fmt : Format) -> String -> interpFormat fmt
toFunction (FInt f) a = \i => toFunction f (a ++ show i)
toFunction (FString f) a = \s => toFunction f (a ++ s)
toFunction (FOther c f) a = toFunction f (a ++ singleton c)
toFunction FEnd a = a

printf : (s : String) -> interpFormat (formatString s)
printf s = toFunction (formatString s) ""

See https://gist.github.com/puffnfresh/6e8f551ea61c5c83b409394760186074 for an example on how to use this function with runtime strings.