Re: some trivial questions?

[Stefan.Lucks@uni-weimar.de]

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On Wed, 1 Nov 2017, tclwarrior@gmail.com wrote:

> i meant to say, why doesn't "use" imply "with" i think every time you 
> insert a use, you will most definitely add a with

Well, in short textbook examples you will find something like

   "with Ada.Text_IO; use Ada.Text_IO",

as the so-called "context clause" (the first few lines of code, with only 
"with" and "use" clauses).

But you should not extrapolate to larger programs. You may have to "with" 
plenty of packages. But if they "use" more than one or two of them
in the context clause, the readability of your program quickly declines. 
It can be quite hard to figure out to which package a certain subprogram 
you are calling belongs to.


It is often better to "use" libraries locally, rather than in context 
clauses. On the other hand, to "use" them (or to use them without a "use" 
clause), you must always "with" them in the context clause. You could say,
the "with"s in the context clause define the dependencies of your program.


Variant 1: use in context clause:

with Ada.Text_IO;
use Ada.Text_IO;

procedure Hello_World is
begin
   Put_Line("Hello World!");
end Hello_World;


Variant 2: "use" local

with Ada.Text_IO;

procedure Hello_World is
   use Ada.Text_IO;
begin
   Put_Line("Hello World!");
end Hello_World;


In thsz example, the difference between context-clause "use" and local 
"use" does not really matter. But if you have a bigger package, it greatly 
improves the readability to "use Ada.Text_IO" only in those subprograms, 
which actually perform textual IO. Same for "use"ing other packages in 
other subprograms, of course.


> i think in a program with a large import (use) list of libraries, this can
> save many lines and looks cleaner

As I worte before: You may have to "with" a long list of libraries, but 
you should never "use" more than one or two of them in the context clause.


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