Generating a random number, not sure why I'm getting this error

Generating a random number, not sure why I'm getting this error

[John Smith]

Hello,

I don't understand why this is failing to compile.  I don't understand the rationale for the failure above all else:

with Ada.Numerics.Discrete_Random;
...
Ada.Numerics.Discrete_Random.Reset(Ada.Numerics.Discrete_Random.Generator);

And this is the error that I get:
:13:15: invalid prefix in selected component "Ada.Numerics.Discrete_Random" -- refers to the first Ada.Numerics.Discrete_Random...

The Ada documentation says that Discrete_Random has a Reset function, so why the error?
https://www2.adacore.com/gap-static/GNAT_Book/html/aarm/AA-A-5-2.html

I think I'm grossly misunderstanding how I'm calling Ada functions inside of packages.

Re: Generating a random number, not sure why I'm getting this error

[Jeffrey R. Carter]

On 11/15/2015 06:25 PM, John Smith wrote:
> 
> with Ada.Numerics.Discrete_Random; ... 
> Ada.Numerics.Discrete_Random.Reset(Ada.Numerics.Discrete_Random.Generator);

Generator is a type, not something you can pass to a subprogram. What would you
expect a call to Random to return?

> And this is the error that I get:
> 
> :13:15: invalid prefix in selected component "Ada.Numerics.Discrete_Random"
> -- refers to the first Ada.Numerics.Discrete_Random...

Not a great error msg.

Discrete_Random is not a package: it's a generic package. You can consider a
generic like a template for a package or subprogram. You create a package or
subprogram from the generic through what Ada calls instantiation. When you
instantiate Discrete_Random, you supply a discrete subtype to fill in for
Result_Subtype; Random returns a value of this subtype.

So, for example,

package Random is new Ada.Numerics.Discrete_Random (Result_Subtype => Character;
-- Random is the name of the pkg created by the instantiation

Gen : Random.Generator;
C   : Character;

Random.Reset (Gen => Gen);
C := Random.Random (Gen);

-- 
Jeff Carter
"Apart from the sanitation, the medicine, education, wine,
public order, irrigation, roads, the fresh water system,
and public health, what have the Romans ever done for us?"
Monty Python's Life of Brian
80

Re: Generating a random number, not sure why I'm getting this error

[John Smith]

On Sunday, November 15, 2015 at 9:29:06 PM UTC-5, Jeffrey R. Carter wrote:
> On 11/15/2015 06:25 PM, John Smith wrote:
> > 
> > with Ada.Numerics.Discrete_Random; ... 
> > Ada.Numerics.Discrete_Random.Reset(Ada.Numerics.Discrete_Random.Generator);
> 
> Generator is a type, not something you can pass to a subprogram. What would you
> expect a call to Random to return?
> 
> > And this is the error that I get:
> > 
> > :13:15: invalid prefix in selected component "Ada.Numerics.Discrete_Random"
> > -- refers to the first Ada.Numerics.Discrete_Random...
> 
> Not a great error msg.
> 
> Discrete_Random is not a package: it's a generic package. You can consider a
> generic like a template for a package or subprogram. You create a package or
> subprogram from the generic through what Ada calls instantiation. When you
> instantiate Discrete_Random, you supply a discrete subtype to fill in for
> Result_Subtype; Random returns a value of this subtype.
> 
> So, for example,
> 
> package Random is new Ada.Numerics.Discrete_Random (Result_Subtype => Character;
> -- Random is the name of the pkg created by the instantiation
> 
> Gen : Random.Generator;
> C   : Character;
> 
> Random.Reset (Gen => Gen);
> C := Random.Random (Gen);
> 
> -- 
> Jeff Carter
> "Apart from the sanitation, the medicine, education, wine,
> public order, irrigation, roads, the fresh water system,
> and public health, what have the Romans ever done for us?"
> Monty Python's Life of Brian
> 80

Ok, that's what I was missing.  I didn't realize that that was how generic packages should be used.  I figured that they should be used just like any other package.

Thank you.

Re: Generating a random number, not sure why I'm getting this error

[Simon Wright]

John Smith <yoursurrogategod@gmail.com> writes:

> I didn't realize that that was how generic packages should be used.  I
> figured that they should be used just like any other package.

See for example https://en.wikibooks.org/wiki/Ada_Programming/Generics -
also http://www.adaic.org/learn/materials/