package ppx_map

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A PPX rewriter to simplify the declaration of maps


Dune Dependency






ppx_map is a PPX rewriter to simplify the definition of maps.


Simple cases

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.


let a = 0 in
[%map 1 => "one"; a => "zero"; 2 => "two"]

will work just fine.

More complex cases

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"]

You can also use functors!

[%map Functor (Module); key => value]

But they 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!