Steve Smith wrote:
On 22/07/15 19:20, Paul van Helden wrote:
I have never managed to understand why "declare before use" is so
important
to Pascal. I get the bit about strong typing and doing everything
possible
to eliminate errors at compile time, but I still don't get why. If
this is
central to what makes something "Pascal", I would love to be pointed to a
good explanation. I'm not trying to be a pain here. I've been programming
in Pascal, full time, for most of my life. It has always puzzled me why
everything has to be declared first.
Consider the following code:
procedure Test;
var i : integer;
begin
for J := 1 to 25 do
writeln;
end;
With "Declare before use", this is plainly incorrect. J has not been
declared; You can't type; You are an idiot!
Since the value of J is undefined at the normal exit of a for loop, one
might be tempted to allow implicit definition. But what about this case:
for j := 0 to 24 do
if j = 12 then
break
else
WriteLn;
What is j here, and what would it be if an implicit j were defined which
only had the scope of the loop?
The whole issue of variable declarations appearing before begin is
central to the distinction between ALGOL (and derivatives including C)
and Pascal (and derivatives including Modula). A related issue is
whether a variable's type appears before (ALGOL, C) or after (Pascal,
Modula) its name.
Another issue is whether if...then is followed by a single statement
(ALGOL 60, Pascal) or by multiple statements followed by fi or end
(ALGOL 68, Modula). And then there's x := if...then...else...; which
exists in ALGOL and (in a more compact form) in C, but not in Pascal and
its derivatives.
And those are the shibboleths by which Pascal are recognised.
--
Mark Morgan Lloyd
markMLl .AT. telemetry.co .DOT. uk
[Opinions above are the author's, not those of his employers or colleagues]
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