The magazine of the Melbourne PC User Group REBOL — An Overview Trevor Gosbell

Function Names

Literal Words

Blocks

Words, making up both program code and data, are collected into blocks. Blocks are delimited by square brackets, and can be assigned to a variable name just like any other value.

For example, the following works just as well as the previous definition of the "add-up" function:

Blocks are the main structural feature of REBOL scripts.

Network Protocols

A useful collection of Network protocols is seamlessly integrated into the language, including HTTP, FTP, SMTP, POP and TCP. A good demonstration of the simplicity of REBOL's Networking capability is this one-line e-mail sender:

This prints out the HTML source of the given Web page:

Dialects

One of the most exciting features of the language is the ability to virtually rewrite the language using "relative expressions" or "dialects". This can help simplify otherwise complex tasks. Here is an example using the Visual Interface Dialect (VID) that is resident in REBOL/View:

It puts the Melb PC logo in a window on your screen.

I will return to REBOL/View in a later article. Third party programmers have also written other dialects including ones for HTML, XML and Flash.

Evaluation Of Expressions

There are no operator precedence rules in REBOL - all processing proceeds left-to-right. For example:

gives the result 90 not 70 as primary school mathematics dictates. Use brackets or reorder the operators to make the intention clear. This does not present as much of a problem as you would think at first.

An example Script - A Batch File Renamer

All of the previous code samples in this article can be tried at the REBOL command line. Now I present a short but complete REBOL script (see Listing 1 below).

Firstly every REBOL script has a header, enclosed in the square brackets [ ], that includes some descriptive information, followed by the code of the script itself. Comments are embedded in the code, preceded by a semi-colon.

Enter the script into a text editor (or download it from http://members.melbpc.org.au/~tgosbell/) and save it as rebname. This is a command line utility - at the command line usage is:

For example, I may have a set of documents called letter-draft.doc, budget- draft.xls, presentation-draft.ppt, and so on. To change the drafts to final versions, I can enter:

And the job is done.

In the next instalment, I will outline how to use REBOL/View to make a graphical front end for the renamer.

 REBOL [

  Title: "REBOL rename"

  Date: "March 2003"

  Name: "rename"

  Version: 0.1

  File: "rebname"

  Author: "Trevor Gosbell"

  Owner: "Trevor Gosbell"

  Rights: "Open to Melb PC members"

  Usage: {rebol rename "find-pattern" ["replace-pattern"]}

  Purpose: {For all file names in the current directory, replaces find-pattern 

with replace-pattern.

If replace-pattern is omitted, find-pattern is removed from the file names.}

  Comment: {It CHANGES FILE NAMES - use at your own risk!}

] 

; retrieve command line arguments from the system object and convert to a block

cl_args: make block! system/script/args 

; if the first argument exists, assign it to the search term

search_term: either none? cl_args/1 [""] [to-string cl_args/1] 

; if the second argument exists, assign it to the replace term

replace_term: either none? cl_args/2 [""] [to-string cl_args/2] 

; change working directory to start up directory (change-dir system/options/path)

; read all of the filenames in working directory (read)

; loop through all of the files in the directory (foreach file)

foreach file read change-dir system/options/path

[

  ; process files only, not directories

  if not dir? file

  [

    ; only proceed with this file if the search term is found

    if not none? find file search_term

    [

      ; make a new file name by replacing the search term with the replace term

      new_file: to-file replace to-string file search_term replace_term 

      ; if the filename already exists, don't rename the current file

      either exists? new_file

      [

        ; print a warning

        print ["FILENAME" new_file "ALREADY IN USE"]

      ]

      [ 

        ; print notification

        print ["Renaming" file "to" new_file] 

        ; rename the file (finally!)

        rename file new_file

      ]

    ]

  ]

] 

; exit the REBOL environment when finished

quit 

      

Listing 1. A Batch File Renamer  (This listing can be downloaded directly from this link.)

About the Author
Trevor Gosbell's first program - a rude variation on "Hello World" - was saved to paper tape in 1981. However it wasn't until the late 1990s that he began making a living from programming, as a programmer and project manager on Web database projects for a Sydney based software house. He is currently a Web site content manager at a well-known Melbourne educational institution. In his spare time he writes programs and writes about programming.

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