When comparing APL vs Rebol, the Slant community recommends Rebol for most people. In the question“What is the best programming language to learn first?” Rebol is ranked 55th while APL is ranked 66th. The most important reason people chose Rebol is:
Almost natural language, for example: `write %out.html read http://google.com`.
Ranked in these QuestionsQuestion Ranking
No complicated loop processing to apply a function to a array of arrays. Functions are defined in a way that they will typically operate the same way on any number of array dimensions. This, along with the clear syntax, leads to very compact code that can be comprehended in a single line, rather than spread out over many pages.
Pro Clear syntax
There is no operator precedence to memorize, as everything is evaluated right-to-left. E.g., in APL 3*10+3 = 39. You do have to type in some otherwise unusual characters, such as ↓ and ∊, but those are easy enough to pick up -- and they have the advantage of being easily remembered once understood, as they often have some connection to common mathematical symbols.
You can seriously implement Conway's Game of Life in one line. There's a reason we do algebra with symbols instead of story problems. APL is good as a language of thought, since you can hold entire algorithms in your head at once.
Pro Human friendly
Almost natural language, for example:
write %out.html read http://google.com.
Code is data, and data can be code. Rebol is based on a simple block data structure, used both for data and for the code itself. Blocks can be manipulated programmatically, and then evaluated as code. This makes metaprogramming easy in Rebol.
Pro Graphical user interface
Beginners are usually stuck making command-line applications in other languages, because GUIs are too hard. Rebol GUIs are easy enough to start with.
Pro Very simple syntax
Rebol's name came from "Relative Expression Based Object Language". Rebol is a functional language and everything is an expression that returns a value. Things that have to be baked into the grammar in other languages are simple function calls with block arguments in Rebol.
Pro Domain specific languages
Rebol's simple homoiconic syntax makes it easy to create "Rebol dialects"--domain-specific languages optimized for a particular purpose. The Rebol distribution includes many of these, and users are free to create more. These DSLs make tasks that would be complicated to express in other languages easy.
Con Does not prepare you for most of the practical programming languages of today
While APL does have a strong use in certain areas (mostly mathematically intensive applications), it is a Domain-Specific language. That along with the fact that its syntax is not similar to C-like or other common syntax forms means that learning APL and expecting it to help you with learning other languages is like learning Calculus and expecting it to make English easier.
APL symbols are only used by APL. You have to learn how to type them and how to read them. It doesn't work well with standard text editors , version control systems, search engines, or web forums. This makes it difficult for a beginner to find help.
Con Write-only language
Maybe you can learn to read it with experience. And an interpreter. Reading APL is like reading a college math book. You might have to study a single line for fifteen minutes to understand what it's doing. And that's if you're an expert at APL. This also applies if you wrote it yourself more than a month ago. hopefully you have comments.
Con Flawless diamond
You can't extend the language itself. (J does this better.) Of course, what's built in is quite powerful.
Con No separators
A function call expression has no parentheses, and there are no separators between sequenced expressions, not even newlines. This means that you have to know the arity of every function in an expression to know how to parse it. It also means you can run into subtle, hard-to-find bugs if you don't provide enough arguments, since the result of the next expression will be passed instead.
Con Not (yet?) Free Software
Rebol 2's core is free (gratis) for commercial use, but the license prohibits modification, a violation of the DFSG. Rebol 3 is Free Software (Apache 2.0), but isn't production ready.