Re: Gnat bug or mistaken program?

["Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org>]

On 7/21/21 4:23 AM, Richard Iswara wrote:
> On 20/07/2021 21.09, Niklas Holsti wrote:
>> On 2021-07-20 15:02, Richard Iswara wrote:
>>> I get this error on my program from the menu build => check semantic
>>> exponent must be of type Natural, found type "Standard.Float".
>>>
>>> Should not 10 ** (log10 a + log10 b) = a*b?
>>>
>>> This is my compiler build:
>>>
>>> GNAT Studio Community 2020 (20200427) hosted on x86_64-pc-mingw32
>>> GNAT Community 2020 (20200429-93) targetting x86_64-pc-mingw32
>>> SPARK Community 2020 (20200429)
>>>
>>> Here is the relevant part of program:
>>>
>>> with Ada.Numerics.Elementary_Functions;
>>> use Ada.Numerics;
>>
>>
>> To make Ada.Numerics.Elementary_Functions."**" visible without qualification, 
>> you should also do "use Ada.Numerics.Elementary_Functions".
>>
>> Otherwise the compiler will see only the predefined operator:
>>
>>     function "**"(Left : Float; Right : Integer'Base) return Float
>>
>> (see RM 4.5.6(9 and 10)) which explains why the compiler does not accept a 
>> floating-point value as the Right operand to "**".
>>
>>
>>> Logs : Float := 0.0;
>>> Multiples : Float;
>>     ...> Multiples := 10.0 ** Logs;  => this is where it fails
>>
>>
>> It fails because the compiler sees only the predefined "**" operator which has 
>> an integral right operand.
> 
> Thank you. So it is a visibility problem.

"Understanding visibility is the key to understanding Ada." -- /Ada Distilled/

Recommending the use package clause as a solution to a misunderstanding of 
visibility is a disservice to a beginning user. Widespread application of use 
pkg clauses is a crutch to avoid understanding visibility. I recommend that 
those who do not understand visibility avoid the use clause altogether, as this 
forces them to learn about visibility. When one understands visibility, one can 
then make reasoned decisions about whether and when to add use clauses.

In decreasing order of specificity, the ways to call an operation in a pkg are

* Use the full name: Ada.Numerics.Elementary_Functions."**" (10.0, Logs)
   This calls the operation once without changing its visibility
* Rename the operation:
   function "**" (Left : Float; Right : Float) return Float renames
      Ada.Numerics.Elementary_Functions."**";
   This makes the specific operation visible
* Use type: this makes all operators of the type visible (not applicable
   in this case)
* Use all type: this makes all operations of the type visible (not
   applicable in this case)
* Use package: this makes everything in the package visible

Use pkg is clearly overkill for this case, and overuse of it can have negative 
consequences.

-- 
Jeff Carter
"Choose a data representation that
makes the program simple."
Elements of Programming Style
188