From 2208b7c7f457bbba9bf030e25de7c27f67901d48 Mon Sep 17 00:00:00 2001
From: NeilBrown <neil@brown.name>
Date: Wed, 8 May 2019 20:50:30 +1000
Subject: [PATCH] oceani: add "and then" and "or else"

"and then" and "or else" are short-circuit versions
of "and" and "or" - they only evaluate second arg if it
is needed.

Signed-off-by: NeilBrown <neil@brown.name>
---
 csrc/oceani-tests.mdc | 14 +++++++++++++
 csrc/oceani.mdc       | 46 ++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 57 insertions(+), 3 deletions(-)

diff --git a/csrc/oceani-tests.mdc b/csrc/oceani-tests.mdc
index f5f097a..b2c2e1b 100644
--- a/csrc/oceani-tests.mdc
+++ b/csrc/oceani-tests.mdc
@@ -157,6 +157,15 @@ Now we need to test if/else and some different loops
 			count = count + 1
 		print "error is ", target - guess * guess
 
+		for j:=0; then j = j+3 ; while j < 10:
+			if j != 0 and then 20 / j > 3:
+				print "20 /", j," =", 20 / j
+			else:
+				print "I won't calculate 20 /", j
+		pi ::= 3.1415926535897
+		if 355/113 == pi or else +(pi - 355/113) < 0.001:
+			print "Close enough"
+
 ###### output: cond_loop
 	Success
 	 1 2 4 8 16 32 64
@@ -165,6 +174,11 @@ Now we need to test if/else and some different loops
 	2 1.41422
 	3 1.41421
 	error is  -4.51095e-12
+	I won't calculate 20 / 0
+	20 / 3  = 6.66667
+	20 / 6  = 3.33333
+	I won't calculate 20 / 9
+	Close enough
 
 ## Say Hello
 
diff --git a/csrc/oceani.mdc b/csrc/oceani.mdc
index dd02e81..123fffc 100644
--- a/csrc/oceani.mdc
+++ b/csrc/oceani.mdc
@@ -1867,12 +1867,16 @@ Our first user of the `binode` will be expressions, and particularly
 Boolean expressions.  As I haven't implemented precedence in the
 parser generator yet, we need different names for each precedence
 level used by expressions.  The outer most or lowest level precedence
-are Boolean `or` `and`, and `not` which form an `Expression` out of `BTerm`s
-and `BFact`s.
+are Boolean operators which form an `Expression` out of `BTerm`s and
+`BFact`s.  As well as `or` `and`, and `not` we have `and then` and `or
+else` which only evaluate the second operand if the result would make
+a difference.
 
 ###### Binode types
 	And,
+	AndThen,
 	Or,
+	OrElse,
 	Not,
 
 ###### Grammar
@@ -1885,6 +1889,13 @@ and `BFact`s.
 			b->right = $<3;
 			$0 = b;
 		} }$
+		| Expression or else BTerm ${ {
+			struct binode *b = new(binode);
+			b->op = OrElse;
+			b->left = $<1;
+			b->right = $<4;
+			$0 = b;
+		} }$
 		| BTerm ${ $0 = $<1; }$
 
 	BTerm -> BTerm and BFact ${ {
@@ -1894,6 +1905,13 @@ and `BFact`s.
 			b->right = $<3;
 			$0 = b;
 		} }$
+		| BTerm and then BFact ${ {
+			struct binode *b = new(binode);
+			b->op = AndThen;
+			b->left = $<1;
+			b->right = $<4;
+			$0 = b;
+		} }$
 		| BFact ${ $0 = $<1; }$
 
 	BFact -> not BFact ${ {
@@ -1910,11 +1928,21 @@ and `BFact`s.
 		printf(" and ");
 		print_exec(b->right, -1, 0);
 		break;
+	case AndThen:
+		print_exec(b->left, -1, 0);
+		printf(" and then ");
+		print_exec(b->right, -1, 0);
+		break;
 	case Or:
 		print_exec(b->left, -1, 0);
 		printf(" or ");
 		print_exec(b->right, -1, 0);
 		break;
+	case OrElse:
+		print_exec(b->left, -1, 0);
+		printf(" or else ");
+		print_exec(b->right, -1, 0);
+		break;
 	case Not:
 		printf("not ");
 		print_exec(b->right, -1, 0);
@@ -1922,7 +1950,9 @@ and `BFact`s.
 
 ###### propagate binode cases
 	case And:
+	case AndThen:
 	case Or:
+	case OrElse:
 	case Not:
 		/* both must be Tbool, result is Tbool */
 		propagate_types(b->left, c, ok, Tbool, 0);
@@ -1940,11 +1970,21 @@ and `BFact`s.
 		right = interp_exec(b->right);
 		rv.bool = rv.bool && right.bool;
 		break;
+	case AndThen:
+		rv = interp_exec(b->left);
+		if (rv.bool)
+			rv = interp_exec(b->right);
+		break;
 	case Or:
 		rv = interp_exec(b->left);
 		right = interp_exec(b->right);
 		rv.bool = rv.bool || right.bool;
 		break;
+	case OrElse:
+		rv = interp_exec(b->left);
+		if (!rv.bool)
+			rv = interp_exec(b->right);
+		break;
 	case Not:
 		rv = interp_exec(b->right);
 		rv.bool = !rv.bool;
@@ -3587,7 +3627,7 @@ Fibonacci, and performs a binary search for a number.
 		a : number
 		a = A;
 		b:number = B
-		if a > 0 and b > 0:
+		if a > 0 and then b > 0:
 			while a != b:
 				if a < b:
 					b = b - a
-- 
2.43.0