$boolEval: what changed

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Title: $boolEval

New		Line
1	+	The Free Pascal compiler [[local compiler directives\|compiler directive]]
2	+
3	+	``` pascal
4	+	{$boolEval}
5	+	```
6	+
7	+	determines, whether short-circuit evaluation (also known as “lazy evaluation”) of [[Boolean]] expressions is performed.
8	+
9	+	## Explanation
10	+
11	+	[[Standard Pascal\|Pascal]] defines, that terms of logical expressions are always evaluated from start to finish. However, in certain cases of expressions containing \[\[And\\|
12	+
13	+	``` pascal
14	+	and
15	+	```
16	+
17	+	\]\] or \[\[Or\\|
18	+
19	+	``` pascal
20	+	or
21	+	```
22	+
23	+	\]\] further evaluation can be skipped, since the final result can be deduced from the intermediate result, thus saving computation time. This is similar to shell’s
24	+
25	+	``` bash
26	+	&&
27	+	```
28	+
29	+	and
30	+
31	+	``` bash
32	+	\|\|
33	+	```
34	+
35	+	. But Pascal consciously decided against such behavior. Ratio: Expressions can consist of function calls, those returning a
36	+
37	+	``` pascal
38	+	boolean
39	+	```
40	+
41	+	value, and if their definitions trigger side-effects a short-circuit evaluation could prevent such being executed. That is contrary to Pascal’s paradigm, that the programmer is not required to know of (implicit) optimizations (that actually change a program’s behavior), but what she writes is what is being executed. The order one writes terms in is not supposed to matter, since $`\land`$ and $`\lor`$ are associative operators, too.
42	+
43	+	The following two examples are semantically identical:
44	+
45	+	``` pascal
46	+	{$push}
47	+	{$boolEval off} // enable lazy evaluation
48	+	if itIsLate(now) and isTired(me) then
49	+	{$pop}
50	+	begin
51	+	brushTeeth;
52	+	goToBed;
53	+	end;
54	+	```
55	+
56	+	But this requires the reader to know
57	+
58	+	``` pascal
59	+	isTired
60	+	```
61	+
62	+	is possibly not called, if
63	+
64	+	``` pascal
65	+	itIsLate
66	+	```
67	+
68	+	returned \[\[false and true\\|
69	+
70	+	``` pascal
71	+	false
72	+	```
73	+
74	+	\]\]. Such implicit “removal” of code, that the order one writes expressions in is relevant, is depreciated by Pascal. Instead one writes:
75	+
76	+	``` pascal
77	+	if itIsLate(now) then
78	+	begin
79	+	if isTired(me) then
80	+	begin
81	+	brushTeeth;
82	+	goToBed;
83	+	end;
84	+	end;
85	+	```
86	+
87	+	By default the [[FPC]] uses shortcut Boolean evaluation. [[Extended Pascal]], which as of 2020 FPC plans to support one day, defines the additional logical operators
88	+
89	+	``` pascal
90	+	and_then
91	+	```
92	+
93	+	and
94	+
95	+	``` pascal
96	+	or_else
97	+	```
98	+
99	+	which do exactly this, lazy evaluation, but on the explicit request of the programmer.
100	+
101	+	## See also
102	+
103	+	- \[<https://www.freepascal.org/docs-html/prog/progsu4.html>
104	+	``` pascal
105	+	{$B}
106	+	```
107	+
108	+	or
109	+
110	+	``` pascal
111	+	{$boolEval}
112	+	```
113	+
114	+
115	+	complete Boolean evaluation in “Free Pascal programmer's guide”\]
116	+	- \[<https://www.freepascal.org/docs-html/rtl/math/ifthen.html>
117	+	``` pascal
118	+	math.ifThen
119	+	```
120	+
121	+	\], the mock ternary operator that always computes both possible results.
122	+
123	+	---
124	+	From the Free Pascal wiki, page "$boolEval", as it stood on 2020-06-23 after a change by Kai Burghardt, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's [[Licensing]] page. If you have concerns about this material, [message the admin](/wiki-contact).
125	+	Category: Local compiler directives

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