support for global variables

1 files changed, 28 insertions, 0 deletions

diff --git a/interpreter.py b/interpreter.py
index fd327f9..9a375d0 100644
--- a/interpreter.py
+++ b/interpreter.py
@@ -1,5 +1,6 @@
 from parser import Expr
 
+ENVIRONMENT = {}
 
 def interpret(exprs):
     ret = None
@@ -10,6 +11,13 @@ def interpret(exprs):
 def evaluate(expr):
     if isinstance(expr, Expr.Literal):
         return expr.value
+    elif isinstance(expr, Expr.Symbol):
+        # defined symbols will evaluate to their expression,
+        # undefined will return their own name
+        # TODO this is bad
+        if not expr.name in ENVIRONMENT:
+            raise Exception(f"no such symbol: {expr}")
+        return interpretEnv(expr, ENVIRONMENT[expr.name])
     elif expr.symbol.name == "not":
         return interpretNot(expr)
     elif expr.symbol.name in ("*", "/"):
@@ -33,6 +41,11 @@ def evaluate(expr):
     elif expr.symbol.name == "print":
         return interpretPrint(expr)
 
+    # global variables
+    elif expr.symbol.name == "def":
+        return interpretDef(expr)
+
+
     else:
         raise Exception(f"unable to evaluate: {expr}")
 
@@ -160,3 +173,18 @@ def interpretPrint(expr):
         raise Exception("'print' takes zero or one argument")
 
     return None  # print returns nothing
+
+def interpretDef(expr):
+    if len(expr.args) != 2:
+        raise Exception("'def' requires a name and an expression")
+    if not isinstance(expr.args[0], Expr.Symbol):
+        raise Exception("'def' requires a string literal as a name")
+    name = expr.args[0].name  # NOTE: we are not evaluating the name!!
+    if not isinstance(name, str):
+        raise Exception("'def' requires a string literal as a name")
+
+    ENVIRONMENT[name] = expr.args[1]
+    return None
+
+def interpretEnv(expr, env_expr):
+    return evaluate(env_expr)  # TODO more than this