Re: Iterators (was Re: some questions re. Ada/GNAT from a C++/GCC user)

[fjh@mundook.cs.mu.OZ.AU (Fergus Henderson)]

mikey@mcs.com (Mike Young) writes:

>eachus@spectre.mitre.org> says...
>>The choice was between Ada and a shell script...I chose Ada:
>>
>>generic
>>  with procedure To_Do(File: in String);
>>procedure Iterate_Files(Pattern: in String := "*";
>>                        Directory: in String := "");
>>-- This generic iterates over all files in a directory matching Pattern and
>>-- calls To_Do once for each such directory entry.  If a file has several
>>-- links in the directory To_Do will be called once for each.
>
>=========
>I don't know about that, Robert. The equivalent shell script would be:
>
>#---- iterateFiles
>fileop=$1;shift
>for file in $*; do $fileop $file; done

No, that shell script, like the vast majority of shell scripts around,
is buggy.  Writing correct, portable shell scripts is actually pretty tricky.

Here's a better version:

	fileop="$1"; shift
	case $# in
		0) ;;
		*) for file in "$@"; do $fileop "$file"; done ;;
	esac

Now just be careful with the quoting when you invoke it...

> You are not restricted to procedures in that same program, all system
> utilities are directly available, you don't need to rebuild to add new
> features, and you need not have bothered with your 50 lines of code.
> All are big wins, IMO.

Oh, certainly shell scripts are extremely useful. 
But it helps to be aware of their drawbacks as well as their advantages.

--
Fergus Henderson <fjh@cs.mu.oz.au>   |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>   |  of excellence is a lethal habit"
PGP: finger fjh@128.250.37.3         |     -- the last words of T. S. Garp.