[Haskell-cafe] Sneaky method for var-arg fns?

[Mateusz Kowalczyk]

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On 26/07/13 22:08, Micah Cowan wrote:
> So, just for fun, I came up with a way to abuse the language in
> order to define a "function" whose argument is optional:
> 
>> -- dirty-trick.hs - A sneaky way to do var args fns in Haskell
>> 
>> {-# LANGUAGE FlexibleInstances #-}
>> 
>> class Hello a where hello :: a
>> 
>> instance Hello (String -> String) where hello = (\str -> "Hello "
>> ++ str)
>> 
>> instance Hello String where hello = hello "World"
> 
> In ghci,
> 
> putStrLn $ hello
> 
> gives "Hello World", while
> 
> putStrLn $ hello "There"
> 
> gives "Hello".
> 
> .
> 
> I was wondering if there was a way to do it in "pure" Haskell
> (i.e., no GHC pragmas required), and also the specific reason for
> why the above example doesn't work without the pragma (I think it's
> just that in general a -> b is not syntactically allowed for type
> specifiers within instance declarations)?
> 
> I'm also interested in alternative approaches to creating 
> variable-argument functions, if there are any.
> 
> .
> 
> If anyone's curious, this was prompted by a discussion with a
> friend (copied), about Haskell and Clojure. He mentioned that
> Clojure can accept variable arguments, though AFAICT all Clojure
> functions basically act like they take a list (that supports
> variable types), so accepting an empty argument list is a bit
> analogous to Haskell accepting an empty list, rather than no
> arguments.
> 
> Part of the reason Haskell can't really take "variable arguments"
> is that all Haskell functions really just take one argument. But I
> figured you could use the contextually expected type to decide
> whether to return a simple value (not /technically/ a function in
> that case), or a function expecting further arguments, which could
> then be extended to define a function taking any arbitrary number
> of arguments.
> 
> Cheers! -mjc
> 
> _______________________________________________ Haskell-Cafe
> mailing list Haskell-Cafe at haskell.org 
> http://www.haskell.org/mailman/listinfo/haskell-cafe
> 
You might be interested in Oleg's
http://okmij.org/ftp/Haskell/polyvariadic.html

- -- 
Mateusz K.
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