Re: Why .ads as well as .adb?

[Maciej Sobczak <see.my.homepage@gmail.com>]

> On the other hand, I like it that I can get a first impression of an Ada 
> program by reading just the .ads files.

This. This is important.
I have seen the suggestion to treat spec files as a low-level requirements document, to be completed by the body. This is a convincing argument for keeping these files separate.

But on the other hand, there are coding patterns that achieve the same effect without distinguishing between spec and implementation. The interface design pattern does it, for example. This is exactly how Java programmers achieve decoupling - but note that this is a voluntary action, so can be also limited to only those places where it is actually needed.

Say, I need this idiom in 10% of packages. Why am I penalized in the remaining 90% and forced to write specs that are demonstrably not needed?

In particular, it is very difficult to explain this concept to beginners, because in simple examples the redundancy of syntax is just shocking - the package spec looks like copy-paste of signatures; and arguably this is how some of the specs are born in practice. The language that forces me to copy-paste stuff between files is not inviting me to study it more deeply.

What we are discussing is whether this copy-pasting has added values that justify the costs.

> Apart from all the other reasons already given, keeping spec and body in 
> the same file can cause compilation-order problems.

Only if you assume that the compiler reads the file only once and therefore must validate everything in a single pass.

Which is not the case anyway, not even in Ada (nor in C++). That's why this concept looks a bit archaic today.

-- 
Maciej Sobczak * http://www.inspirel.com