On Fri, 4 Dec 2020, Ben Franksen wrote:
I am writing an interpreter for a very simple untyped language and I want to provide a built-in function to format a list of values in a printf like fashion. There is the beautiful Text.Printf module with its multi-variadic printf function, and I would like to use that to do all the heavy lifting.
I can easily implement an
instance PrintfArg Value
for my Value type, but what I need in addition to that is an "untyped" version of printf i.e. something like
format :: String -> [Value] -> String
with the property that
format fmt [] = printf fmt format fmt [x] = printf fmt x format fmt [x,y] = printf fmt x y ...
{-# LANGUAGE Rank2Types #-} import Text.Printf (PrintfType, printf) type Value = Int formatWith :: (forall t. (PrintfType t) => t) -> [Value] -> String formatWith pf [] = pf formatWith pf (x:xs) = formatWith (pf x) xs format :: String -> [Value] -> String format fmt = formatWith (printf fmt) *Main> format "(%d,%d)" [1,2] "(1,2)" *Main> format "(%d,%d)" [1,2,3] "(1,2)*** Exception: printf: formatting string ended prematurely *Main> format "(%d,%d)" [1] "(1,*** Exception: printf: argument list ended prematurely