Where is the ambiguity in my grammar?

Question

I have the following ambiguous grammar where the capital letter rules are for simple lexical tokens:

program : function | function program
function : FN ID ARROW TYPE invariants DASH block
fn_call : ID L_PAREN CONSTANT R_PAREN


invariants : invariant | invariant invariants
invariant : TYPE ID COLON invariant_tests
invariant_tests : test | test COMMA invariant_tests
test : OPERATOR CONSTANT | STAR


block : expression | expression block
expression : loop | fn_call | assignment | def | constant
loop : FOR def IN CONSTANT COLON block
assignment : ID ASSIGN CONSTANT | ID ASSIGN ID
def : TYPE assignment

I receive 8 shift/reduce errors (when using Python 2.7 and PLY).

Is the ambiguity a result of how I define "one or more", eg. block : expression | expression block?

Answer 1

As a previous speaker said, you must differentiate between the "block usages". For example, try:

loop : FOR def IN CONSTANT COLON block END FOR

Answer 2

The problem was how I defined the loop:

loop : FOR def IN CONSTANT COLON block

since the parser didn't understand if the block belonged to the loop or to the function.

Basically, another variant of the dangling else problem.