ppx_map is a PPX rewriter to simplify the definition of maps.
If the type of your map keys is simple enough (bool, char, float, int, string or unit) and the PPX can deduce it, it is as simple as:
[%map 0 => "zero"; 1 => "one"; 2 => "two"]which will give something similar to:
let module Int_map = Map.Make (Int) in
Int_map.(empty |> add 0 "zero" |> add 1 "one" |> add 2 "two")The extension is able to automatically type the map if the first key is a non-bound value (e.g. not defined by a let) of the types given above. For example,
let a = 0 in
[%map a => "zero"; 1 => "one"; 2 => "two"]will give the following compilation error:
Error: `map' cannot infer the type of this value. You need to give an explicit
bool, char, float, int, string or unit.
whereas
let a = 0 in
[%map 1 => "one"; a => "zero"; 2 => "two"]will work just fine.
While it may seem trivial, creating an empty map requires a little more than just [%map]; a type must be specified:
[%map.Int]or
[%map Int]will do the trick. I don’t know which syntax I prefer; pick your own and stick to it!
If the first key you give is a bound value, you need to help the rewriter a little:
let (a, b, c) = (0, 1, 2) in
[%map.Int a => "zero"; b => "one"; c => "two"]or
let (a, b, c) = (0, 1, 2) in
[%map Int; a => "zero"; b => "one"; c => "two"]Again, you can decide which syntax you prefer.
You can also use functors! But only the second syntax is going to work:
[%map Functor (Module); key => value]These functors need to be of arity 1 (Functor (Module) (Module') cannot be used as it wouldn’t work well with OCaml’s parser). As we could expect, using a generative functor gives the following compilation error:
Error: This expression has type 'a $Map.t
but an expression was expected of type 'b
The type constructor $Map.t would escape its scope
Don’t do that!
Also, I don’t really see why you’d ever need to use functors here, but that was fun to implement 🙂