jihchi / let-anything

let%Anything

This is a ppx that looks for let%Anything, where Anything is a valid module identifier (e.g. captialized), and turns it into Anything.let_(value, pattern => continuation).

It's an exploration of this PR to the reason repo.

Here's an example of using it (source)

let%OptIf () = id != store.data.root;
let%Opt node = get(store, id);
let%Opt parent = get(store, node.parent);
Some((
  [NodeChildren(parent.id, parent.children |. List.keep((!=)(node.id))), DeleteNode(node.id),
  View({...store.view, active: nextId})
  ],
  [Event.Node(parent.id), Event.View(Node(nextId))]
))

try%Anything

If you have a monad with a "failure" case that you want to be able to handle, try%Anything expr { | exn => ... } is translated into Anything.try_(expr, function { | exn => ...}). So, for a real example:

let%Result value = try%result (getName()) {
  | Failure(message) => Error("Unable to get name: " ++ message)
  | DefaultName => Ok("Lorraine")
  | _ => Error("Unknown error getting name")
};

And here are the modules used (source)

module Opt = {
  let let_ = (a, b) => switch (a) {
    | None => None
    | Some(x) => b(x)
  }
};

module OptIf = {
  let let_ = (a, b) => if (a) {
    b()
  } else {
    None
  }
};

module Result = {
  let let_ = (value, fn) => switch value {
    | Ok(v) => fn(v)
    | Error(_) => value
  };
  let try_ = (value, fn) => switch value {
    | Ok(_) => value
    | Error(err) => fn(err)
  };
}