BNFC generated grammar fails on simplest examples

Question

I would like to write an interpreter in haskell for a simple imperative language. To do that, I first wrote gramar of that language for the tool BNFC (http://bnfc.digitalgrammars.com/).

Part of that grammar is dedicated to arithmetic expressions, such as:

EAdd.     Expr ::= Expr "+" Expr ;
EMinus.   Expr ::= Expr "-" Expr ;
EMul.     Expr ::= Expr "*" Expr ;
ENum.     Expr ::= Integer ;

Having just that, I can run the BNFC tool and test it by provided script. It parses arithmetic operations succesfully.

However, if I add another section (let's say with types):

Tint.    Type ::= "int" ;

And then put the expr secion, arithmetic operations no longer parse (when testing on 1 + 2 it says "Parse failed... [some tokens here] syntax error at line 1 before 1 + 2")

Why does it happen? How to fix it?

To rephrase:

Why such gramar:

TInt.  Type ::=  "int" ;
EAdd.  Expr ::= Expr "+" Expr ;
ENum.  Expr ::= Integer ;

does not parse correctly 1 + 1 using bnfc?

Grégoire Grégoire · Accepted Answer · 2016-06-05T14:56:01

In the absence of an entrypoint declaration, bnfc will use the first category defined in the grammar as entry point in the test script.

I.e. if you add Tint. Type ::= "int" ; at the top of your file, the script generated by bnfc will try to parse a Type, not an Expr.

BNFC generated grammar fails on simplest examples

1 Answers