Introduction

Almost any reader of this Journal knows what Parsing is about. One of the strengths of Perl is its excellence for text analysis. Additionally to its embedded regular expression capacities, modules like Parse::RecDescent [1] and Parse::Yapp [2] make easier the task of text understanding and text transformation. This is in clear contrast with the absence of Perl 5 generic tools¹giving support for the subsequent stages of text processing. The exception being the module Language::AttributeGrammar [3]. Parrot does well in this chapter, having the Parrot Grammar Engine (PGE) [4] and the Tree Grammar Engine (TGE) [5].

Parse::Eyapp (Extended yapp) is a collection of modules that extends Francois Desarmenien Parse::Yapp 1.05: Any yapp program runs without changes with eyapp. Additionally Parse::Eyapp provides new functionalities like named attributes, EBNF-like expressions, modifiable default actions, abstract syntax tree building and translation schemes. It also provides a language for tree transformations. This article introduces the basics of translator construction with Parse::Eyapp through an example that compiles infix expressions into Parrot Intermediate Representation (PIR)[6]. The input to the program will be a (semicolon separated) list of infix expressions, like:

1  b = 5;
2  a = b+2;
3  a = 2*(a+b)*(2-4/2); # is zero
4  print a;
5  d = (a = a+1)*4-b;
6  c = a*b+d;
7  print c;
8  print d

and the output is an equivalent PIR:

 1  .sub 'main' :main
 2     .local num a, b, c, d
 3     b = 5
 4     a = b + 2
 5     a = 0 # expression at line 3 
 6     print "a = "     # above was
 7     print a    # reduced to zero
 8     print "\n" # at compile time
 9     a = a + 1
10     $N5 = a * 4
11     d = $N5 - b
12     $N7 = a * b
13     c = $N7 + d
14     print "c = "
15     print c
16     print "\n"
17     print "d = "
18     print d
19     print "\n"
20  .end