Most other programming languages don't have an end if statement required at the end of conditional statements.
if boolean_expression then statement else statement
What advantage does including an end if provide Ada that other languages don't have?
if boolean_expression then statement else statement end if
I think it has something to do with branch prediction if the condition is true (skips the else portion of the code), but I'm not sure.
Consider a nested if, like this: if boolean_expression then if another_boolean_expression then statement else statement
Now, to which if statement does the else part belong?
end if resolves the issue (indented for clarity):
if boolean_expression then
if another_boolean_expression then
statement
end if;
else
statement
end if;
or:
if boolean_expression then
if another_boolean_expression then
statement
else
statement
end if;
end if;
It carries no special advantage, but it is a necessary part of any language that allows block declarations.
Ada's method of block termination is very explicit, but that does not convey any particular advantage.
As the others are indicating, it is simply Ada's method of block termination. The syntactic detail is not very important, but the semantics are critical.
Consider a few examples of how block termination is expressed in other languages:
end for lambdas and conditionals, the full-stop character . for function definitions, and , and ; for phrase segregation.end. Very similar to Ada.if is terminated with fi, and case terminates with esac.{ and close all blocks with a closing } which allows the block's closing token to be easily identified (by a relatively naive parser, not necessarily a human reader).( (which essentially means "begin evaluation") while lists are opened with a '( (which essentially means "I'm a list, but skip evaluating me") and all are closed with a closing paren ).<foo> is terminated by a matching tag with a closing slash indicator like </foo> (or sometimes just </>, which is slightly better for important reasons). Most variants of angle-bracket madness now support "self-closing" tags as well, where the closing slash can exist at the end of the opening tag as a shorthand, so an empty tag that is its own phrase like <foo></foo> can be shortened to <foo /> or sometimes <foo/> depending on what flavor of insanity it happens to be. (Obviously there are a few semantic black holes one can be sucked into with this sort of ambiguity.)Etc...
As we can see, Ada's block termination expression isn't in any way odd -- it is just more explicit than the brackety languages (and slightly more explicit than Erlang and Prolog). Regardless, without resorting to GOTOs or other forms of explicit jump expression it is absolutely necessary to be able to define logical blocks and that means also having a clear way of terminating them. This is just Ada's way.
As flyx has kindly pointed out, there is an interesting distinction among statements, blocks, expressions, procedures, subroutines, and functions. Some languages have all of these, some have only functions and expressions, some have any arbitrary mix.
C (and its descendants) make a distinction between "statements" and "expressions", and in the case of a C-style block delimited by {} braces the language treats that "block" as a single statement. Meaning, it is not expected to return a value the way an expression would, it is expected that execution of the procedure represented by the statement itself will have some meaningful effect on the program.
So what does that mean in terms of syntax?
In Ada the if explicitly opens a block.
In C, however, an if doesn't open or declare anything at all -- it simply guards the next statement. So a one-line if in C is perfectly normal to see -- no brackets need be closed at all because none were opened. If, however, the statement following the if needs to be a multi-line statement (note that it is a bit unidiomatic in C to define functions for every multi-line statement).
if (x) {do_something();}
is the same as
if (x) do_something();
Look, Ma, no brackets!
The former is more common as most conditional branches are more than a single line.
This particular issue becomes a bit more cloudy (and now you actually do get into branch prediction issues) with the C ternary operator:
int opening_time = (day == SUNDAY) ? 12 : 9;
or Python's somewhat recently added ternary operator:
opening_time = 12 if day == 'sunday' else 9
An equivalent (in somewhat unidiomatic C):
int opening_time = 9;
if (day == SUNDAY) opening_time = 12;
And so on.
Here is where we get into understanding both some of the very early utility of un-named blocks of code, and how in some (but not all) languages they are treated as compound statements.
{} for blocks, the if statement is defined as treating the (one) succeeding statement as the code that gets executed only when the condition is met. So when you want to execute multiple statements conditionally, you use a block as the statement following the if because a block is a statement. That's pretty different to Ada where the if statement automatically starts a block. –
Subtotal if in C and C-like languages at all -- and that indeed demands some explanation. –
Etch if, and/or a label, can help the compiler decide where a block terminator is missing. This isn't always decidable with curly brackets. –
Neuron Ada language has been designed with concern to readability and to code defect prevention. So scope closing constructs are mandatory and correspond to scope opening constructs. When you have a long piece of code it would be easier to figure out what that closing entry does actually close.
procedure Foo is
begin
if condition then
-------------------
-- a lot of code...
-------------------
end if;
end Foo;
In contrast some other languages with different syntax may leave it up to developers whether to write additional comments to clarify closing entry purpose or whether to write closing entry at all (C++ example):
void
Foo(void)
{
if(condition)
{
///////////////////
// a lot of code...
///////////////////
} // if
} // Foo
Such code turns to be either less readable and potentially prone to scoping issues or requires extra efforts from the developer.
© 2022 - 2024 — McMap. All rights reserved.
ifblocks, use characters (such as{and}) to delimit blocks, or have something equivalent toend if. – Poignant