[Haskell-cafe] fromInteger for Lists

[Ryan Ingram]

I wish for a language extension "OverloadedLiterals" that includes all of these:

1 => fromInteger 1
"hello" => fromString "hello"
['a', 'b', 'c'] => cons 'a' (cons 'b' (cons 'c' nil))
[ ord x | x <- ['a', 'b', 'c'], ok x ]
  =>
cons 'a' (cons 'b' (cons 'c' nil)) >>= \x -> guard (ok x) >> return (ord x)


Maybe something similar for pair notation too!

GHC already has:

class Num a where ... fromInteger :: Integer -> a
class IsString where fromString :: String -> a

What is the right interface for cons/nil?  John's fromList is one
suggestion.  I'd suggest either:

class ListLike c where
   type Element c
   fromList :: [Element c] -> c

or

class Nil c where
   nil :: c
class Container c e where
   cons :: e -> c -> c

This would allow something like:

import qualified Data.Map as M

data KeyVal k v = k :> v
instance Ord k => ListLike (M.Map k v) where
   type Element (M.Map k v) = k :> v
   fromList = foldr insertElem M.empty where
       insertElem (k :> v) m = M.insert k v m

which lets you have "first-class" looking maps:
   M.lookup "Bob" ["Bob" :> 1, "Alice" :> 2, "John" :> 15]

  -- ryan

On Fri, May 1, 2009 at 4:06 PM, John Dorsey <haskell at colquitt.org> wrote:
> Paul,
>
>> There's nothing better than making a data type an instance of Num. In
>> particular, fromInteger is a joy. But how about lists?
>
> Do you mean something like Blargh below, only useful?
>
> John
>
>
> dorsey at elwood:~/src/scratch$ cat list.hs
>
> class Blargh f where
>  fromList :: [a] -> f a
>
> data Foo a = Foo [a] deriving (Show)
> data Bar a = Bar [a] deriving (Show)
>
> instance Blargh Foo where
>  fromList = Foo
>
> instance Blargh Bar where
>  fromList l = Bar (reverse l)
>
> dorsey at elwood:~/src/scratch$ ghci list.hs
> GHCi, version 6.8.3: http://www.haskell.org/ghc/  :? for help
> Loading package base ... linking ... done.
> [1 of 1] Compiling Main             ( list.hs, interpreted )
> Ok, modules loaded: Main.
> *Main> fromList [1,2,3] :: Foo Int
> Foo [1,2,3]
> *Main> fromList [1,2,3] :: Bar Int
> Bar [3,2,1]
> *Main>
>
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