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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: surprise data from Ada.Sequential_IO
Date: Mon, 22 Mar 2021 22:54:22 +0200
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On 2021-03-22 21:41, Jeffrey R. Carter wrote:

> The only differences between Direct_IO and Sequential_IO are that 
> Direct_IO allows random access and mixed input and output. In both cases 
> the file contains a sequence of binary representations of values of a 
> single type.
> 
> I don't really understand why Sequential_IO exists, since Direct_IO 
> provides a superset of Sequential_IO's functionality.

Not quite. From the RM:

    generic
       type Element_Type is private;
    package Ada.Direct_IO is
    ...

and

    generic
       type Element_Type(<>) is private;
    package Ada.Sequential_IO is
    ...

So Sequential_IO allows unknown discriminants in the element type, in 
other words elements that can vary in size, which obviously hampers 
random (indexed) access to elements in the file, which Direct_IO allows.

There is also this note for Direct_IO:

RM A.8.4(19.a): Reason: The Element_Type formal of Direct_IO does not 
have an unknown_discriminant_part (unlike Sequential_IO) so that the 
implementation can make use of the ability to declare uninitialized 
variables of the type.