Add substr_ macro

[tueda]

Although this may be a "feature creep" and I'm not sure whether many people find it useful or not, I would like to make a "Work in Progress" pull request, hoping this may stimulate other discussions.

As far as I know, there is no way/trick to extract a part of a string in FORM. So I experimentally implemented substr_(STRING,POS,LEN) macro, which is more or less substr of Perl and PHP but the first character is at POS=1. (Note that it is not substring_(STRING,FROM,TO)). POS and LEN are somewhat extended to non-positive and negative integers, respectively. Especially, LEN=-1 means the actual length of STRING.

An application is defining a procedure with optional parameters:

#procedure Proc1(x,?opts)
  #define expand "-1"
  #do opt={`?opts'}
    #ifdef `opt'
      #switch `opt'
        #case expand
          #redefine expand "0"
          #break
        #case noexpand
          #redefine expand "-1"
          #break
        #default
          #if "`substr_(`opt',1,7)'" == "expand="
            #redefine expand "`substr_(`opt',8,-1)'"
          #else
            #message Error: illegal option of Proc1: `opt'
            #terminate
          #endif
          #break
      #endswitch
    #endif
  #enddo

  #message x=`x'
  #message expand=`expand'
#endprocedure

which accepts #call Proc1(x,expand=2) as well as #call Proc1(x), #call Proc1(x,expand) or #call Proc1(x,noexpand).

This branch also contains an implementation of strlen_(STRING), but it is not primitive in the sense that it can be implemented based on substr_:

#procedure StrLen(?str,dest)
  #redefine `dest' "0"
  #do loop=1,1
    #if "`substr_(`?str',1,``dest'')'" != "`?str'"
      #redefine `dest' "{``dest''+1}"
      #redefine loop "0"
    #endif
  #enddo
#endprocedure

#define n ""
#call StrLen(12345,n)

Other string operations like strreplace_(STRING,OLD,NEW) can also be implemented based on substr_.

[tueda]

FYI: if your pull request is still incomplete and needs more work, then there is the "Draft" feature for pull requests in GitHub.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/changing-the-stage-of-a-pull-request

Even if your work is in progress, creating a draft pull request to get others' feedback sometimes be beneficial (I have changed the status of this pull request to "Draft").

[tueda]

Note: I found 1d4b775 added takeleft_, takeright_, keepleft_ and keepright_.

Example:

#message `takeleft_(abcdefg,2)'
#message `takeright_(abcdefg,2)'
#message `keepleft_(abcdefg,2)'
#message `keepright_(abcdefg,2)'
.end

    #message `takeleft_(abcdefg,2)'
~~~cdefg
    #message `takeright_(abcdefg,2)'
~~~abcde
    #message `keepleft_(abcdefg,2)'
~~~ab
    #message `keepright_(abcdefg,2)'
~~~fg