The following syntax is used for the text input of the Guarded Commands with Procedures Code, starting with a command C.

D ::=

var x | array A[n+] | proc p(VL; VL) C end | D; D

C ::=

x := a | {D; C} | p(AL; VL) | A[a] := a | skip | C; C | if GC fi | do GC od

VL ::=

x | x, VL

AL ::=

a | a, AL

GC ::=

b -> C | GC [] GC

a ::=

n | x | A[a] | a + a | a - a | a * a | a % a | a / a | -a | a ^ a | (a)

b ::=

true | false | b & b | b | b | b && b | b || b | !b | a = a | a != a | a > a | a >= a | a < a | a <= a | (b)

Additional notes:

• && and || do short-circut evaluation (i.e. a minimal number of operands are evaluated left to right), the rest of the operators do eager evaluation (i.e. all operands are evaluted).
• Precedence and associativity rules:
  • Precedence is highest for - (unary minus), then ^, then *, % and /, and lowest for + and -.
  • Precedence is highest for !, then & and &&, and lowest for | and ||.
  • *, %, /, +, -, &, |, &&, and || associate left to right.
  • !, ^, - (unary minus), [], and ; associate right to left.
• Variables x, y, z, arrays A and procedures p match the regular expression [a-zA-Z][a-zA-Z\d_]* and can not be any of the keywords. Furthermore the three sets should be pair-wise disjoint within a scope, e.g. a variable identifier can not be used both as a variable and an array (and a procedure) in the same scope.
• Numbers n match the regular expression \d+ and are bound by 2⁵³-1 (JavaScript Number.MAX_SAFE_INTEGER). Negative numbers are passed as unary minus -n.
• Procedures p can only be called if they are declared.
• Positive numbers n+ match the regular expression [0]*[1-9][0-9]*, and are bound like numbers n.
• A whitespace matches the regular expression [ \u00A0\f\n\r\t\v], with a mandatory whitespace after if␣, do␣, proc␣, before ␣fi, ␣od, ␣end. Whitespaces are ignored anywhere else.
• Procedure calls must have as many input and output parameters as their declaration specifies.