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In school we have been studying metalanguages, in particular, railroad diagrams and EBNF. I received a question where an imaginary programming language (winston) was described in EBNF. Here it is: 

Digit = 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
LCase = a | b | c | d
UCase = A | B | C | D | E | F | G | H | I | J
Operator = + | - | * | /
Logical = < | > | <= | >= | <>
Constant = [-] <Digit>{<Digit>}
Identifier = <UCase>{<LCase> | <Digit>}
Assignment = Set <Identifier> to <Constant> | <Identifier
                    {<Operator>(<Constant> | <Identifier>)}
Condition = <Identifier> <Logical> (<Identifier> | <Constant>)
                    {(and | or) <Identifier> <Logical> (<Identifier> | <Constant>)}
When = (<Assignment> |  <Condition> {<Assignment> | <Condition>})
Statement = <Input> | <Output> | <Assignment> | <Condition> | <When> | <Pretest> | <Posttest>
Program = Start <Statement> {! <Statement>} Stop

The program written below was made with winston but doesn't execute properly. Use the EBNF descriptions to identify the error.

Start
Input J1
Input J2
When (J1 = J2, Set A3 to 0), (J1 < J2, Set A3 to -1), Set A3 to 1
Output A3
Stop

My working so far: To me, this program seems legitimate. It is a program so if must start with "start" and end with "stop", which it does. The statements in the middle seemingly are allowed to be in there. Can someone point me in the right direction?

Also, can someone tell me what it means in the EBNF description of a program what this means:<statement>

I think it means statements like when and if but Im not too sure. Thanks for the help :)

Andrea
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Jaskirat Boparai
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1 Answers1

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When is comma-separated and the grammar doesn't specify commas at all.

J1 = J2 -- there is no = comparison op in the grammar (see Logical), so J1 = J2 is neither Assignment, nor Condition and is thus invalid.

<statement> -- the grammar wraps symbols in angle brackets on the right-hand side, e.g. Identifier on the left-hand and, later, <Identifier> in Assignment rule -- that doesn't look like a valid EBNF.

rns
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