Re: Procedure Problem

[Jim Langston <jlangston@xxxxxxxxxxxxxxxx>]

Now we get into something I think is not good.

I abhor a function or subroutine to change the contents of a passed variable.

And yes, that is common too, but I think it is much worst than having to
pass another variable.

As a user of a function, I have to be very aware that a function is changing
one or more of my variables.  If I am not aware of this, I can trash my data
very quickly.

Consider a function that returns a value and changes a variable.  Such as
something like (in pseudo C syntax)

/* Function to return the length of a string without leading or trailing spaces
*/
int StringLength(char *SomeVar);

Perhaps as a side effect this function modifies SomeVar in some way.
Such as Trimming it to get the length without spaces.  If I am not aware
of this I can do somethign like this:

MyVar = '  SomeText';
MyVarLen = StringLength(MyVar);
Write(OutFile, MyVar);             /* Write the string to file */
Write(OutFile, MyVarLen);       /* Write the TRIMed length to file */

Now, not being aware that StringLength changed MyVar and made it
'SomeText'.  I now wrote 'SomeText8' or the such to the file (in my
make believe Write statement).  I thought I wrote '  SomeText8'.

I know that, yes, I should be aware of any side effects of any functions
I use.  But, this is not always the case.

And, yes, I have been bitten by this one before.  Very difficult bug
to track down.

PC languages tend to be getting away from this by returning structures
and not just variables.  So StringLength would return a structure of

struct StringLengthrReturn
  int TrimmedLength;
  char *TrimmedString;

or the such, then it doesnt' have to modify the variable.

Regards,

Jim Langston

John Taylor wrote:

> Jim,
>
> <SNIP>
> If you want use a function call, as in Jon's example:
>
>     Eval    NiceChar = Center(%Trim(MessyChar): %Size(NiceChar))
>
> you're going to have to pass the size of receiving variable. To avoid this,
> I just write the routine as a procedure call, so that the centering is done
> within the parameter that it's passed in. For example:
>
>     Eval    NiceChar = MessyChar
>     CallP  Center( NiceChar )

<SNIP>

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