Add syntax for creating finite maps and sets

Many languages have syntax for creating maps. Commonly used syntax is either

{key1: value1, key2: value2, ...}

or

{key1 => value1, key2 => value2, ...}

Since few Haskellers (with the notable exception of SPJ) use the curly bracket Haskell syntax for layout, I thought we could co-opt it and instead use it for map (and set literals.) The feature would of course be protected by a language pragma as to not break old code.

Strawman proposal:

{key1: value1, key2: value2, ...} desugars to fromList [(key1, value1), (key2, value2), ...], using a new type class:

class IsMap m where
  type Key m
  type Value m

  fromList :: [(Key m, Value m)] -> m

{:} is the literal for empty maps.

Analogously, {value1, value2, ...} desugars to fromList [value1, value2, ...], using a new type class:

class IsSet m where
  type Item m

  fromList :: [Item m] -> m

{} is the literal for empty sets.

Both the containers and unordered-containers packages would presumably provide instances. It's possible that mutable data structures, such as those provided by the hashtables package, could provide instances as well.

• *Open question**: will this clash badly with the record syntax? It seems to work in other languages which use { and } to define other entities.

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Related	→ #436 (closed)

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Not using the curly braces, but similar can be achieved using OverloadedLists and defining := as pattern for (,) as described here. Then one could write:

myMap :: Map String Int
myMap = ["a" := 1, "b" := 33, "c" := 44]

Not exactly the same but quite close.

(This issue was created ~9 years ago, so perhaps within the same discussion when IsList and OverloadedLists were added?)