CS计算机代考程序代写 # Concrete syntax

# Concrete syntax

A video on this section can be found [here](https://bham.cloud.panopto.eu/Panopto/Pages/Viewer.aspx?id=b173484f-59de-4533-bbdd-ac85012e8bb2).

Here is a grammar for our language, written in [BNF](https://en.wikipedia.org/wiki/Backus%E2%80%93Naur_form), where expressions with lower numbers describe operators with lower precedence.
The grammar is [ambiguous](https://en.wikipedia.org/wiki/Ambiguous_grammar) regarding `else`. We resolve the ambiguity by associating `else`’s to the closest `if`’s to the left.

“`
Program ::= Identifier := Expr;
| { [Program] }
| while (Expr) Program
| if (Expr) Program
| if (Expr) Program else Program

Expr ::= Expr1 | Expr1 OrOp Expr
Expr1 ::= Expr2 | Expr2 AndOp Expr1
Expr2 ::= Expr3 | Expr3 EqOp Expr2
Expr3 ::= Expr4 | Expr4 CompOp Expr3
Expr4 ::= Expr5 | Expr5 AddOp Expr4
Expr5 ::= Expr6 | Expr6 MulOp Expr5
Expr6 ::= Expr7 | NotOp Expr6
Expr7 ::= Constant | Identifier | (Expr)

OrOp ::= ||
AndOp ::= &&
EqOp ::= ==
CompOp ::= <= | < | >= | >
AddOp ::= + | –
MulOp ::= * | / | %
NotOp ::= !
“`
We will use monadic parsing (please read chapter 13 of the book) to convert from concrete syntax to [abstract syntax](AbstractSyntax.md).

#### Next: [Abstract syntax](AbstractSyntax.md)