lens that fails to focus raises no signals

[lanyusan]

This could be bug or just a design feature that should be improved.

🐛 Bug report

When use a lens to modify some data, if the path doesn't exist, lens.modify doesn't do anything but return the existing object.

Is this the right behavior?

I think an exception should be thrown or use either to return a left value indicating a lens has failed to focus.

[gcanti]

Please provide a reproducible example

[lanyusan]

Here you go:

Test code:

import { Lens, fromTraversable, Prism } from "monocle-ts";
import { array } from "fp-ts/lib/Array";

const lensData = {
  a: {
    b: {
      c: {
        d: 3,
      },
    },
    p: [
      {
        id: 1,
        name: "Jack",
      },
      {
        id: 2,
        name: "Jon",
      },
      {
        id: 3,
        name: "Jay",
      },
    ],
  },
};

try {
  const r = Lens.fromProp<any>()("a")
    .composeLens(Lens.fromProp<any>()("p"))
    .composeTraversal(fromTraversable(array)<any>())
    .composePrism(Prism.fromPredicate((child: any) => child.id === 4))
    .modify((child) => ({ ...child, name: "Josh" }))(lensData);

  console.log("Failed prism returned old value silently");
  console.log(JSON.stringify(r, null, 2));

  const r2 = Lens.fromProp<any>()("a")
    .composeLens(Lens.fromProp<any>()("p"))
    .composeTraversal(fromTraversable(array)<any>())
    .composePrism(Prism.fromPredicate((child: any) => child.id === 2))
    .modify((child) => ({ ...child, name: "Josh" }))(lensData);

  console.log("Successfully prism modified value of a -> p -> id = 2 from Jon to Josh");
  console.log(JSON.stringify(r2, null, 2));
} catch (e) {
  console.log("Caught error");
}

run with yarn ts-node test.ts

result with no exception

Failed prism returned old value silently
{
  "a": {
    "b": {
      "c": {
        "d": 3
      }
    },
    "p": [
      {
        "id": 1,
        "name": "Jack"
      },
      {
        "id": 2,
        "name": "Jon"
      },
      {
        "id": 3,
        "name": "Jay"
      }
    ]
  }
}
Successfully prism modified value of a -> p -> id = 2 from Jon to Josh
{
  "a": {
    "b": {
      "c": {
        "d": 3
      }
    },
    "p": [
      {
        "id": 1,
        "name": "Jack"
      },
      {
        "id": 2,
        "name": "Josh"
      },
      {
        "id": 3,
        "name": "Jay"
      }
    ]
  }
}
Done in 0.99s.

[gcanti]

This

const r = Lens.fromProp<any>()("a")
    .composeLens(Lens.fromProp<any>()("p"))
    .composeTraversal(fromTraversable(array)<any>())
    .composePrism(Prism.fromPredicate((child: any) => child.id === 4))

is not a Lens, is a Traversal so is working as expected

[lanyusan]

Ok. I thought they were all called lens in a broader scope.

Can I suggest to make an enhancement to raise a signal if the traversal then modify fails to find the element to update, as in relational database, an update sql could return how many rows have been modified?

It would be nice for some use cases.

Thanks.