Method parseIf

chapter10/src/main/java/com/seaofnodes/simple/Parser.java:315–354

Parses a statement if ( expression ) statement [else statement] @return a Node, never null

()

     * @return a {@link Node}, never {@code null}
     */
    private Node parseIf() {
        require("(");
        // Parse predicate
        var pred = require(parseExpression(), ")").keep();
        // IfNode takes current control and predicate
        Node ifNode = new IfNode(ctrl(), pred).peephole();
        // Setup projection nodes
        Node ifT = new ProjNode(ifNode.  keep(), 0, "True" ).peephole().keep();
        Node ifF = new ProjNode(ifNode.unkeep(), 1, "False").peephole().keep();
        // In if true branch, the ifT proj node becomes the ctrl
        // But first clone the scope and set it as current
        int ndefs = _scope.nIns();
        ScopeNode fScope = _scope.dup(); // Duplicate current scope
        _xScopes.push(fScope); // For graph visualization we need all scopes

        // Parse the true side
        ctrl(ifT.unkeep());     // set ctrl token to ifTrue projection
        _scope.upcast(ifT,pred,false); // Up-cast predicate
        parseStatement();       // Parse true-side
        ScopeNode tScope = _scope;

        // Parse the false side
        _scope = fScope;        // Restore scope, then parse else block if any
        ctrl(ifF.unkeep());     // Ctrl token is now set to ifFalse projection
        if (matchx("else")) {
            _scope.upcast(ifF,pred,true); // Up-cast predicate
            parseStatement();
            fScope = _scope;
        }
        pred.unkeep();

        if( tScope.nIns() != ndefs || fScope.nIns() != ndefs )
            throw error("Cannot define a new name on one arm of an if");

        // Merge results
        _scope = tScope;
        _xScopes.pop();       // Discard pushed from graph display

        return ctrl(tScope.mergeScopes(fScope));
    }


    /**