Mouse (or MousePEG)

A tool to transcribe PEG into an executable parser.

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This is an example of PEG:

Sum     = Space Sign Number (AddOp Number)* !_ {} ;
Number  = Digits? "." Digits Space {}
        / Digits Space {} ;
Sign    = ("-" Space)? ;
AddOp   = [-+] Space ;
Digits  = [0-9]+ ;
Space   = " "* ;

Mouse transcribes it into an executable parser written in Java. Each rule of PEG becomes a Java method named after the rule. For example, the first rule is transcribed into this method;

//=====================================================================
//  Sum = Space Sign Number (AddOp Number)* !_ {} ;
//=====================================================================
private boolean Sum()
  {
    begin("Sum");
    Space();
    Sign();
    if (!Number()) return reject();
    while (Sum_0());
    if (!aheadNot()) return reject();
    sem.Sum();
    return accept();
  }
``

A pair of brackets {} at the end of a PEG rule indicates that you provide a semantic action for the rule. It is a method in a separate Java class and may look like this:

//-------------------------------------------------------------------
// Sum = Space Sign Number AddOp Number ... AddOp Number
// 0 1 2 3 4 n-2 n-1
//-------------------------------------------------------------------
void Sum()
{
int n = rhsSize();
int s = (Double)rhs(2).get();
if (!rhs(1).isEmpty()) s = -s;
for (int i=4;i<n;i+=2)
{
if (rhs(i-1).charAt(0)=='+')
s += (Double)rhs(i).get();
else
s -= (Double)rhs(i).get();
}
System.out.println(s);
}


Special functions like "rhsSize()" and "rhs(i)" provide access to parts of the parsed text.

Tags: langdev   parser   jvm  

Last modified 23 August 2025