initial import of yapps from debian sources

git-svn-id: http://svn.stsci.edu/svn/ssb/etal/exyapps/trunk@356 d34015c8-bcbb-4646-8ac8-8ba5febf221d

1 files changed, 64 insertions, 0 deletions

diff --git a/examples/calc.g b/examples/calc.g
new file mode 100644
index 0000000..5432855
--- /dev/null
+++ b/examples/calc.g
@@ -0,0 +1,64 @@
+globalvars = {}       # We will store the calculator's variables here
+
+def lookup(map, name):
+    for x,v in map:  
+        if x == name: return v
+    if not globalvars.has_key(name): print 'Undefined (defaulting to 0):', name
+    return globalvars.get(name, 0)
+
+def stack_input(scanner,ign):
+    """Grab more input"""
+    scanner.stack_input(raw_input(">?> "))
+
+%%
+parser Calculator:
+    ignore:    "[ \r\t\n]+"
+    ignore:    "[?]"         {{ stack_input }}
+
+    token END: "$"
+    token NUM: "[0-9]+"
+    token VAR: "[a-zA-Z_]+"
+
+    # Each line can either be an expression or an assignment statement
+    rule goal:   expr<<[]>> END            {{ print '=', expr }}
+                                           {{ return expr }}
+               | "set" VAR expr<<[]>> END  {{ globalvars[VAR] = expr }}
+                                           {{ print VAR, '=', expr }}
+                                           {{ return expr }}
+
+    # An expression is the sum and difference of factors
+    rule expr<<V>>:   factor<<V>>         {{ n = factor }}
+                     ( "[+]" factor<<V>>  {{ n = n+factor }}
+                     |  "-"  factor<<V>>  {{ n = n-factor }}
+                     )*                   {{ return n }}
+
+    # A factor is the product and division of terms
+    rule factor<<V>>: term<<V>>           {{ v = term }}
+                     ( "[*]" term<<V>>    {{ v = v*term }}
+                     |  "/"  term<<V>>    {{ v = v/term }}
+                     )*                   {{ return v }}
+
+    # A term is a number, variable, or an expression surrounded by parentheses
+    rule term<<V>>:   
+                 NUM                      {{ return int(NUM) }}
+               | VAR                      {{ return lookup(V, VAR) }}
+               | "\\(" expr "\\)"         {{ return expr }}
+               | "let" VAR "=" expr<<V>>  {{ V = [(VAR, expr)] + V }}
+                 "in" expr<<V>>           {{ return expr }}
+%%
+if __name__=='__main__':
+    print 'Welcome to the calculator sample for Yapps 2.'
+    print '  Enter either "<expression>" or "set <var> <expression>",'
+    print '  or just press return to exit.  An expression can have'
+    print '  local variables:  let x = expr in expr'
+    # We could have put this loop into the parser, by making the
+    # `goal' rule use (expr | set var expr)*, but by putting the
+    # loop into Python code, we can make it interactive (i.e., enter
+    # one expression, get the result, enter another expression, etc.)
+    while 1:
+        try: s = raw_input('>>> ')
+        except EOFError: break
+        if not s.strip(): break
+        parse('goal', s)
+    print 'Bye.'
+