define statement only works if followed by an assignment in Lark grammar

Question

I am creating a parser with Lark. The parser works fine for most of the tests I ran, but failed with the define keyword. It only works if it is followed by an assignement. define a = 10 works just fine, but define b is not treated as a define statement.

Here is the Lark parser :

import lark

# ...

parser = lark.Lark("""
    ?start: statements
    ?statements: ((expr (";" | NEWLINE) | NEWLINE ) )* expr?
    ?expr: identifier | number | functioncall | define | assignment | function
    ?functioncall: identifier "(" arguments? ")"
    ?arguments: expr ("," expr)*
    ?define: "define" identifier ("=" expr)?
    ?assignment: identifier "=" expr

    ?function: "function" "(" parameters? ")" "->" identifier block
    ?parameters: identifier ("," identifier)*
    ?block: "{" statements "}"

    ?identifier: NAME -> identifier
    ?number: NUMBER -> number

    %import common.NEWLINE
    %import common.CNAME -> NAME
    %import common.NUMBER
    %import common.WS_INLINE
    %ignore WS_INLINE
    COMMENT: "/*" /(.|\n)+/x "*/" | "//" /.+/ NEWLINE?
    %ignore COMMENT
""")

My tests :

tree = parser.parse("define a = 10")
assert(tree.data == "define") # OK
tree = parser.parse("define b")
assert(tree.data == "define") # NOT OK - tree.data is "identifier"

Specifically, parser.parse("define b") and parser.parse("b") give the exact same result. I would expect parser.parse("define b") to give a tree beginning with the define rule, but instead I have an identifier.

Answer 1

Sometime Lark parser doesn't clearly identifies a rule, for instance, define b and b gives Tree(identifier, [Token(NAME, 'b')]). To be able to distinguish the two, you need to force Lark to add a name to the rule, this can be done by adding -> name_of_rule at the end of a line in the parser definition. So for instance, the definition of the ?define rule should become:

 ?define: "define" identifier ( "=" expr )? -> define