Function transform 

pub fn transform<F>(expr: Expression, fun: &F) -> Result<Expression>
where
    F: Fn(Expression) -> Result<Option<Expression>>,

Transforms an expression tree bottom-up, with optional node removal.

Recursively transforms all children first, then applies fun to the resulting node. If fun returns Ok(None), the node is replaced with an Expression::Null. If fun returns Ok(Some(expr)), the node is replaced with expr.

This is the primary transformation entry point when callers need the ability to “delete” nodes by returning None.

Example

use polyglot_sql::traversal::transform;

// Remove all Paren wrapper nodes from a tree
let result = transform(expr, &|e| match e {
    Expression::Paren(p) => Ok(Some(p.this)),
    other => Ok(Some(other)),
})?;