Add List-Ops practice exercise

[jojo8356]

Closes #158

Summary

• Add list-ops practice exercise (difficulty 3)
• 8 operations: append, concat, filter, length, map, foldl, foldr, reverse
• Reference solution using pure recursion (no stdlib list functions)
• Test generator with function string → Lean lambda mapping
• 22 test cases from canonical data (integer division for fold tests)

Files

• exercises/practice/list-ops/ — full exercise (stub, tests, example, docs, lakefile)
• generators/Generator/Generator/ListOpsGenerator.lean — test generator
• generators/Generator/Generator.lean — updated dispatch table
• config.json — added exercise entry

[github-actions]

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[keiravillekode]

Next time, please follow How to contribute

[keiravillekode]

This should be your github username

[jojo8356]

Yes I maj with claude because I am in another project with and I am tired, sorry.

[jojo8356]

maj = update

[oxe-i]

Exercises of the same difficulty are organized in alphabetical order. You should use bin/reorder-exercises

[jojo8356]

Done, ran bin/reorder-exercises to fix the ordering. Thanks for the review!

[keiravillekode]

We could define another function ^ for integer power.

  | "foldl", "\"(acc, el) -> el ^ acc\"" => "(fun acc el => el ^ acc)"
  | "foldr", "\"(acc, el) -> el ^ acc\"" => "(fun el acc => el ^ acc)"

We could have a .meta/extra.json like the meetup or atbash-cipher or grep exercises, to compute 3 ^ (2 ^ 5) or 2 ^ (3 ^ 5) using foldl or foldr.

          "input": {
            "list": [2, 3],
            "initial": 5,
            "function": "(acc, el) -> el ^ acc"
          },

[jojo8356]

Done. Added power (^) mappings to the generator and created .meta/extra.json with two extra test cases for foldl/foldr using el ^ acc on [3, 2] with initial 2 (foldl → 512, foldr → 81).
All 24 tests pass.

[keiravillekode]

I don't think you ran the generator.

If we look at some extra.json files, we see they have an array at the top level. We can have

[
  {
    "description": "folds (reduces) the given list from the left with a function -> direction dependent function applied to non-empty list (power)",
    "property": "foldl",
    "input": {
      "list": [2, 5],
      "initial": 3,
      "function": "(acc, el) -> el ^ acc"
    },
    "expected": 390625
  },
  {
    "description": "folds (reduces) the given list from the right with a function -> direction dependent function applied to non-empty list (power)",
    "property": "foldr",
    "input": {
      "list": [5, 3],
      "initial": 2,
      "function": "(acc, el) -> el ^ acc"
    },
    "expected": 1953125
  }
]

The generator can have

  | "foldl", "\"(acc, el) -> el * acc\"" => "(fun acc el => acc * el)"
  | "foldl", "\"(acc, el) -> el + acc\"" => "(fun acc el => acc + el)"

and

  | "foldl", "\"(acc, el) -> el ^ acc\"" => "(fun acc (el : UInt8) => (UInt8.toNat el) ^ acc)"
  | "foldr", "\"(acc, el) -> el ^ acc\"" => "(fun (el : UInt8) acc => (UInt8.toNat el) ^ acc)"

You can run the generator using

cd generators
lake exe generator -g list-ops

[oxe-i]

@jojo8356 It seems you are not running the generator script, as you should, and so the test file does not necessarily match the generator file for the exercise.

Since this PR does not follow our contribution guidelines, it will be closed. Thank you for your contribution.