/migrate

Auto-migrates green-code violations in the current file. Converts bare throw statements to Result<T>, nullable returns to Option<T>, and other violations to their MonadicSharp equivalents.

What it does

1. Runs the same analysis as /forge-analyze on the current file.
2. For each auto-migratable violation, computes a code transformation.
3. Shows a preview diff of all changes before applying anything.
4. Applies changes only when you confirm (or when --dry-run is omitted and you have set autoApply in .monadicleaf.json).

Not every violation is auto-migratable. Violations that require understanding business logic (e.g., choosing an appropriate error type) are flagged for manual review.

Usage

/migrate
/migrate --dry-run
/migrate --rule GC001
/migrate --file src/Services/OrderService.cs

Options

Option	Default	Description
--dry-run	off	Preview changes without writing to disk
--rule	all auto-migratable rules	Limit migration to a single rule code
--file	current file	Override the file to migrate

Preview output (before confirmation)

File: src/Services/OrderService.cs
3 changes ready to apply

──────────────── GC001 — line 42 ────────────────
 Before:
   throw new InvalidOperationException("Order not found");

 After:
   return Result.Fail<Order>(new OrderError.NotFound(orderId));

──────────────── GC003 — line 67 ────────────────
 Before:
   public User? GetByEmail(string email)
   {
       return _db.Users.FirstOrDefault(u => u.Email == email);
   }

 After:
   public Option<User> GetByEmail(string email)
   {
       var user = _db.Users.FirstOrDefault(u => u.Email == email);
       return user is null ? Option.None<User>() : Option.Some(user);
   }

──────────────── GC001 — line 89 ────────────────
 Before:
   catch (DbException ex)
   {
       throw new RepositoryException("Database error", ex);
   }

 After:
   catch (DbException ex)
   {
       return Result.Fail<Unit>(new PersistenceError.DatabaseUnavailable(ex.Message));
   }

Apply these changes? [y/N]

Example: before and after GC001

Before — bare throw propagates as an untyped exception:

public Order GetById(Guid orderId)
{
    var order = _db.Orders.Find(orderId);
    if (order is null)
        throw new InvalidOperationException($"Order {orderId} not found");

    return order;
}

After — error is a value, return type is explicit:

public Result<Order> GetById(Guid orderId)
{
    var order = _db.Orders.Find(orderId);
    if (order is null)
        return Result.Fail<Order>(new OrderError.NotFound(orderId));

    return Result.Ok(order);
}

Auto-migratable rules

Rule	Migration
GC001	throw → return Result.Fail<T>(...)
GC003	T? return → Option<T> return
GC004	bool failable return → Result<T>
GC005	Empty catch {} → explicit error result
GC008	Nullable reference type field → Option<T> field

Rules GC002, GC006, GC007, GC009, and GC010 require context-specific decisions and are not auto-migrated. /migrate will list them and link to the manual migration guide for each.

Configuration

{
  "migration": {
    "autoApply": false,
    "defaultErrorNamespace": "MyApp.Errors",
    "preserveOriginalComments": true
  }
}

autoApply: true skips the confirmation prompt and writes changes immediately. Use with caution.