[Haskell-cafe] untyped printf

[Ben Franksen]

Wow, that simple. Thanks Henning, my untangled brain feels a lot better :-)

Cheers
Ben

Am 04.12.20 um 19:06 schrieb Henning Thielemann:
> 
> 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
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