From 3d533c5a47739af341647b03d0e3ac9b6513fa8e Mon Sep 17 00:00:00 2001
From: Alex Karle <alex@alexkarle.com>
Date: Fri, 18 Nov 2022 11:14:18 -0500
Subject: [PATCH] ch9.2: Implement if statements

Pretty cool to see this shake out :)

> var x = 3;
3
> if (x == 3) { print "woo"; } else { print "else"; }
woo
---
 interpreter.scm | 18 +++++++++++++-----
 parser.scm      | 21 +++++++++++++++++++++
 2 files changed, 34 insertions(+), 5 deletions(-)

diff --git a/interpreter.scm b/interpreter.scm
index 8a50ee4..206abee 100644
--- a/interpreter.scm
+++ b/interpreter.scm
@@ -147,13 +147,21 @@
 	    (begin
 	      (execute (car stmts) new-env)
 	      (loop (cdr stmts)))))))
+   ((if-stmt? stmt)
+    (if (truthy? (evaluate (if-stmt-cond-expr stmt) env))
+	(execute (if-stmt-then-stmt stmt) env)
+	(if (not (null? (if-stmt-else-stmt stmt)))
+	    (execute (if-stmt-else-stmt stmt) env)
+	    '())))
    (else (runtime-err! (format "Unknown stmt ~A" stmt)))))
 
+;; Save the global-env outside interpret so that it persists in the REPL
+(define global-env (make-env #f))
+
 (define (interpret stmts)
   (call/cc (lambda (cc)
 	     (set! interpreter-abort cc)
-	     (let ((global-env (make-env #f)))
-	       (let loop ((sts stmts))
-		 (if (not (null? sts))
-		     (begin (execute (car sts) global-env)
-			    (loop (cdr sts)))))))))
+	     (let loop ((sts stmts))
+	       (if (not (null? sts))
+		   (begin (execute (car sts) global-env)
+			  (loop (cdr sts))))))))
diff --git a/parser.scm b/parser.scm
index dcc6b19..6381f6b 100644
--- a/parser.scm
+++ b/parser.scm
@@ -42,6 +42,7 @@
 (define-record expr-stmt value)
 (define-record var-stmt name init)
 (define-record block stmts)
+(define-record if-stmt cond-expr then-stmt else-stmt)
 
 (set-record-printer! print-stmt
   (lambda (x out)
@@ -59,6 +60,12 @@
   (lambda (x out)
     (fprintf out "(block ~A)" (block-stmts x))))
 
+(set-record-printer! if-stmt
+  (lambda (x out)
+    (fprintf out "(if ~A ~A ~A)"
+      (if-stmt-cond-expr x)
+      (if-stmt-then-stmt x)
+      (if-stmt-else-stmt x))))
 
 
 ;; helper to check if first is of types
@@ -85,6 +92,8 @@
 (define (parse-statement tokens)
   (cond ((top-type? tokens '(PRINT))
 	 (parse-print-statement (cdr tokens)))
+	((top-type? tokens '(IF))
+	 (parse-if-statement (cdr tokens)))
 	((top-type? tokens '(LEFT_BRACE))
 	 (let-values (((stmts toks) (parse-block (cdr tokens))))
 	   ;; TODO: return the block record instead of stmts? Not the
@@ -119,6 +128,18 @@
 	      ;; but (loop) returns multiple values (sigh)
 	      (loop (append stmts (list decl)) rest))))))
 
+(define (parse-if-statement tokens)
+  (if (not (top-type? tokens '(LEFT_PAREN)))
+      (parse-err! tokens "Expected '(' after 'if'")
+      (let-values (((cond-expr toks) (parse-expression '() (cdr tokens))))
+	(if (not (top-type? toks '(RIGHT_PAREN)))
+	    (parse-err! toks "Expected ')' after if condition")
+	    (let-values (((then-stmt toks2) (parse-statement (cdr toks))))
+	      (if (top-type? toks2 '(ELSE))
+		  (let-values (((else-stmt toks3) (parse-statement (cdr toks2))))
+		    (values (make-if-stmt cond-expr then-stmt else-stmt) toks3))
+		  (values (make-if-stmt cond-expr then-stmt '()) toks2)))))))
+
 (define (parse-assignment expr toks)
   (let-values (((e2 t2) (parse-equality expr toks)))
     (if (top-type? t2 '(EQUAL))
--
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