Visitor generation

[LPeter1997]

One of the key features of the Nanopass framework is automating the "boring-parts" of transformation between passes. I think our equivalent could be a generated visitor base class for each pass. By default, it would do a catamorphic transformation on unchanged nodes - that you can still override, if you need -, and would require the user to write the ones that were affected.

For example, let's say we have this AST:

// Pass 1
record Expr
{
    record Unit : Expr;
    record IfElse(Expr Cond, Expr Then, Expr Else) : Expr;
    record If(Expr Cond, Expr Then) : Expr;
    record Name(string Id) : Expr;
}

And then we do the following modifications:

Remove the 'If' node
Add the member 'ISymbol Symbol' to 'Name'
Remove the member 'Id' from 'Name'

Then the next generated AST would look like:

// Pass 2
record Expr
{
    record Unit : Expr;
    record IfElse(Expr Cond, Expr Then, Expr Else) : Expr;
    record Name(ISymbol Symbol) : Expr;
}

And the generated visitor base could be:

abstract class Pass1ToPass2Base
{
    // Generate dispatch functionality
    public Pass2.Expr Apply(Pass1.Expr expr) => expr switch
    {
        Pass1.Expr.Unit e => Apply(e),
        Pass1.Expr.IfElse e => Apply(e),
        Pass1.Expr.If e => Apply(e),
        Pass1.Expr.Name e => Apply(e),
        _ => throw new InvalidOperationException(),
    };

    // Unchanged nodes are just translated trivially
    // The user can still override them, if needed

    public virtual Pass2.Expr Apply(Pass1.Expr.Unit e) => new Pass2.Expr.Unit();

    public virtual Pass2.Expr Apply(Pass1.Expr.IfElse e) => new Pass2.Expr.IfElse(
        Apply(e..Cond),
        Apply(e.Then),
        Apply(e.Else));

    // Changed or removed nodes are required to be handled by the user

    public abstract Pass2.Expr Apply(Pass1.Expr.If e);

    public abstract Pass2.Expr Apply(Pass1.Expr.Name e);
}

Then all the user needs to implement is:

class Pass1ToPass2 : Pass1ToPass2Base
{
    public override Pass2.Expr Apply(Pass1.Expr.If e) => new Pass1.Expr.IfElse(
        Apply(e..Cond),
        Apply(e.Then),
        new Pass2.Expr.Unit());

    public override Pass2.Expr Apply(Pass1.Expr.Name e) => /* ... */;
}