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December 18, 2015 21:49
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Make strings behave like printf.
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| Foo! 23 42 |
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| {-# LANGUAGE OverloadedStrings, FlexibleInstances, UndecidableInstances #-} | |
| {-# LANGUAGE ExistentialQuantification #-} | |
| {-# OPTIONS_GHC -fno-warn-orphans -fno-warn-type-defaults #-} | |
| module Main where | |
| import Data.String ( IsString(..) ) | |
| -------------------- | |
| -- Custom printf that takes '%s' for `Show` values. | |
| -------------------- | |
| class PrintfType t where | |
| spr ::String -> [SomeShow] -> t | |
| instance PrintfType [Char] where | |
| spr format args = uprintf format (reverse args) | |
| instance PrintfType (IO ()) where | |
| spr format args = do | |
| putStr (uprintf format (reverse args)) | |
| instance (Show a, PrintfType r) => PrintfType (a -> r) where | |
| spr format args = \a -> spr format (SomeShow a : args) | |
| data SomeShow = forall a. (Show a) => SomeShow a | |
| uprintf :: String -> [SomeShow] -> String | |
| uprintf "" [] = "" | |
| uprintf "" (_:_) = error "too many arguments; learn to compromise" | |
| uprintf ('%':'%':cs) us = '%' : uprintf cs us | |
| uprintf ('%':_) [] = error "too few arguments; stand up for yourself" | |
| uprintf ('%':'s':cs) (SomeShow n : us) = show n ++ uprintf cs us | |
| uprintf (c:cs) us = c : uprintf cs us | |
| -------------------- | |
| -- Strings are printfs. | |
| -------------------- | |
| instance (PrintfType a) => IsString a where | |
| fromString s = spr s [] | |
| main :: IO () | |
| main = do | |
| "Foo! %s %s\n" 23 42 |
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The associated blog post is here.