Suggestion: Syntactic sugar for Maps!

[Nicolas Frisby]

FWIW, state monad works fine. And I imagine the QuasiQuote extension
could get get rid of the double quotes on strings (and recover the use
of the colon?).


module Sugar ((~>), build, Builder) where

import Data.Map (Map); import qualified Data.Map as Map
import Control.Monad.State

(~>) :: Ord k => k -> a -> Builder k a
k ~> a = do
  m <- get
  let m' = Map.insert k a m
  put m'
  return m'



type Builder k a = State (Map k a) (Map k a)

build :: Builder k a -> Map k a
build x = evalState x Map.empty



m = build $ "zero" ~> 0 >> "one" ~> 1 >> "two" ~> 2

n = build $ do
      "zero" ~> 0
      "one" ~> 1
      "two" ~> 2

o = build $ do { "zero" ~> 0; "one" ~> 1; "two" ~> 2 }


On Sun, Dec 21, 2008 at 7:28 PM, Wolfgang Jeltsch
<g9ks157k at acme.softbase.org> wrote:
> Am Sonntag, 14. Dezember 2008 15:35 schrieb Neil Mitchell:
>> I am fairly certain someone could write the necessary magic so:
>>
>> do {'a' ~> 1; 'b' ~> 2}
>>
>> becomes a map without any changes to the language at all. It seems
>> like throwing syntax at a problem should be a last resort. I often do:
>>
>> let (*) = (,) in ['a' * 1, 'b' * 2]
>>
>> I find that quite elegant as an associative list, which you can then
>> convert to a map, a hash table, use as an associative list etc.
>>
>> I also think that those who are looking for Haskell will have their
>> mind so blown by lazy evaluation that keeping their maps similar isn't
>> so necessary :-)
>>
>> Thanks
>>
>> Neil
>
> +1
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