Re: [Strasheela-devel] can't understand tutorial lesson: $ marks a return value

[Torsten A. <tor...@gm...>]

Dear Graham,


On Dec 27, 2007, at 1:11 AM, Graham Percival wrote:
> Procedures and Functions -> $ marks a return value
>
> Sorry, I can't figure out what this example is showing -- what
> does it mean that "every procedure argument be a return value" ?
> If this is covered in the Roy book, could I get a pointer to the
> section?

Check the index for `$ (nesting marker)', e.g., p. 83 (printed book), =20=

and p. 85 (in pdf draft). I just copied this section here into the mail.

* Nesting markers

The nesting marker =93$=94 turns any statement into an expression. The =20=

expression=92s
value is what is at the position indicated by the nesting marker. For =20=

example, the
statement {P X1 X2 X3} can be written as {P X1 $ X3}, which is an =20
expression
whose value is X2. This makes the source code more concise, since it =20
avoids having
to declare and use the identifier X2. The variable corresponding to =20
X2 is hidden
from the source code.
Nesting markers can make source code more readable to a proficient =20
program-
mer, while making it harder for a beginner to see how the code =20
translates to the
kernel language. We will use them only when they greatly increase =20
readability.
For example, instead of writing:
local X in {Obj get(X)} {Browse X} end
we will instead write {Browse {Obj get($)}}. Once you get used to =20
nesting
markers, they are both concise and clear. Note that the syntax of =20
procedure
values as explained in Section 2.3.3 is consistent with the nesting =20
marker syntax.



>
> In other words, why does this example use
>    X =3D {Max $ 3 Result}
> instead of
>    {Max X 3 Result}

These two lines are equivalent. This is just a toy-example, we should =20=

find something more convincing.

Primarily, I introduce the nesting marker in the tutorial, because I =20
use it all the time to return values from methods like

   {MyNote getPitch($)}


However, once the reader understands what $ actually does, then also =20
the definition of anonymous procedures, functions, classes etc. make =20
sense, e.g.

   fun {$ X} X*X end

Best
Torsten


> ?
>
> Cheers,
> - Graham
>
>
> /* Every procedure argument can be a return value.  The $ (dollar
>  * sign)
> always marks a return value.  This example is not yet very
> convincing,
> but procedures will encapsulate constraints in later examples...
> TODO:  what's happening here?  -gp
> TODO: I think this belongs somewhere in Basics-2, or even in the
> CSP stuff.
> */
>
> local
>    proc {Max X Y Z}
>       if X >=3D Y then Z =3D X else Z =3D Y end
>    end
>    X =3D 4
>    Result
> in
>    %% Note the $
>    X =3D {Max $ 3 Result}
>    {Browse Result}
> end
>
>
>
> ----------------------------------------------------------------------=20=

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--
Torsten Anders
Interdisciplinary Centre for Computer Music Research
University of Plymouth
Office: +44-1752-233667
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