[janino-dev] [jira] Commented: (JANINO-116) Need a Java.StatementList for more flexibility in generating ASTs

    [ http://jira.codehaus.org/browse/JANINO-116?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=136367#action_136367 ]

Matt Fowles commented on JANINO-116:
------------------------------------

Consider the following block of code which is fairly idiomatic of things done in our system.

{code}
public CValue take(StmtContainer block, final CValue value) {
    //take lazily
    final CValue[] taken = new CValue[schema.getFieldCount()];
    final CValue[] values = new CValue[schema.getFieldCount()];
    for(Field f : schema.getFields()) {
        values[f.getIndex()] = createSubValue(value, f);
    }
       
    //use a statement sequence so we inject the code here
    //instead of some later place in the block when ref() gets called
    final CG.StmtSequence stmtSeq = new CG.StmtSequence();
    block.add(stmtSeq);
       
       
    return new CValue(this) {
        @Override
        public Expr ref(int i) {
            if(i == NULL_REF) {
                return value.ref(i);
            }
            int offset = getFieldOffsetForRef(i);
            int remainder = getFieldRefForRef(i);
            if(taken[offset] == null) {
                taken[offset] = values[offset].take(stmtSeq);
            }
            return taken[offset].ref(remainder);
        }
    };
}
{code}

The contract for this method only allows it to insert values into block at the current position (if it were to put them intervening code could change the value of things upon which it depends).  However, we won't necessarily need all of the sub values that could be computed by this block.  Hence the method inserts a place holder item into the block, so that it can always inject code at this point.  Then it lazily fills in the code when it knows which values are needed.

I believe that this sort of a feature would be more generally useful to anyone targeting the Janino AST, and it really does not add much complexity to the Janino code base.

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