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What is an "expression language"? I just started doing Java EE crap and there is an assortment of "ELs" that you can embed in your java apps. Outside of Java,

by mahmud 15y ago

What is an "expression language"? I just started doing Java EE crap and there is an assortment of "ELs" that you can embed in your java apps. Outside of Java, an "expression language" doesn't make any sense to me. An expression, statement, binding, assignment, application, and abstraction are all constructs, or elements of programming languages. Most of them can be implemented using others, sure, but I expect all useful languages to be capable of all. So, my question is, what makes an expression language an expression language, to the exclusion of all other constructs? (IOW, why is that particular part being made into a defining characteristic of the language?)

fexl 15y ago
:) It's a function expression language, meaning a language for expressing pure functions of an arbitrary nature. It's really just a variant of the lambda calculus, and I compile those expressions into combinators to eliminate all variables. For example, the "flip" function for swapping the order of two functions is this: \flip = (\x\y y x) But Fexl converts that into pure combinators like so: \flip = (S (C (S I)) C) Actually it uses the higher level combinators L, R, I, etc. so that flip is defined as: \flip = (L I) But the higher-level are shorthand for forms that use S and C only.
- mahmud 15y ago
  Well, these things exist in the wild, and they're not as fun & cute as yours: http://en.wikipedia.org/wiki/Expression_Language http://en.wikipedia.org/wiki/Expression_Language http://docs.jboss.org/seam/latest/reference/en-US/html/elenhancements.html http://docs.jboss.org/seam/latest/reference/en-US/html/elenh... http://java.sun.com/products/jsp/reference/techart/unifiedEL.html http://java.sun.com/products/jsp/reference/techart/unifiedEL...
  - fexl 15y ago
    I hear ya. I consider Fexl to be at the extreme end of computational abstraction. The kind of things you reference are convenient tools for making certain lower-level things accessible from a high level sort of script. I've used that approach in my work for many years with great success. I use ultra-simple domain-specific languages consisting of this syntax: token token token token ... :) Seriously though. It's amazing what you can do in a script consists of nothing but tokens or "words", e.g. do_this 4 5 do_that x "Hello there." Where all filler such as #-comments and white space, even line breaks, are completely insignificant. I've made a lot of hay out of languages like that. I even have a Turing-complete language in that form, and I can define "verbs" which access any sorts of machine functions I care to provide. It ends up looking a bit like Forth but I think more clear (coming from a biased judge of course). I'm actually tending to favor that token-based Turing-complete language over Fexl for doing real work. I'm not devoted to the idea of functional programming, I'm devoted to simple, powerful, flexible, and secure programming. Strangely, the token-based languages make my application code more "language independent" -- meaning that I can write the interpreter in any language I want (C, Perl, etc.) and it really does not matter at all. I like to keep pushing as much application logic into the scripting language as possible. With the Turing-complete language it might even be feasible to do all of my application logic in it, leaving me with only a small residual interpreter written in C. But it remains to be seen if my Turing-complete token language can really scale well to that large body of application logic.

by mahmud 15y ago

fexl 15y ago

:) It's a function expression language, meaning a language for expressing pure functions of an arbitrary nature. It's really just a variant of the lambda calculus, and I compile those expressions into combinators to eliminate all variables. For example, the "flip" function for swapping the order of two functions is this: \flip = (\x\y y x) But Fexl converts that into pure combinators like so: \flip = (S (C (S I)) C) Actually it uses the higher level combinators L, R, I, etc. so that flip is defined as: \flip = (L I) But the higher-level are shorthand for forms that use S and C only.

mahmud 15y ago
Well, these things exist in the wild, and they're not as fun & cute as yours: http://en.wikipedia.org/wiki/Expression_Language http://en.wikipedia.org/wiki/Expression_Language http://docs.jboss.org/seam/latest/reference/en-US/html/elenhancements.html http://docs.jboss.org/seam/latest/reference/en-US/html/elenh... http://java.sun.com/products/jsp/reference/techart/unifiedEL.html http://java.sun.com/products/jsp/reference/techart/unifiedEL...
- fexl 15y ago
  I hear ya. I consider Fexl to be at the extreme end of computational abstraction. The kind of things you reference are convenient tools for making certain lower-level things accessible from a high level sort of script. I've used that approach in my work for many years with great success. I use ultra-simple domain-specific languages consisting of this syntax: token token token token ... :) Seriously though. It's amazing what you can do in a script consists of nothing but tokens or "words", e.g. do_this 4 5 do_that x "Hello there." Where all filler such as #-comments and white space, even line breaks, are completely insignificant. I've made a lot of hay out of languages like that. I even have a Turing-complete language in that form, and I can define "verbs" which access any sorts of machine functions I care to provide. It ends up looking a bit like Forth but I think more clear (coming from a biased judge of course). I'm actually tending to favor that token-based Turing-complete language over Fexl for doing real work. I'm not devoted to the idea of functional programming, I'm devoted to simple, powerful, flexible, and secure programming. Strangely, the token-based languages make my application code more "language independent" -- meaning that I can write the interpreter in any language I want (C, Perl, etc.) and it really does not matter at all. I like to keep pushing as much application logic into the scripting language as possible. With the Turing-complete language it might even be feasible to do all of my application logic in it, leaving me with only a small residual interpreter written in C. But it remains to be seen if my Turing-complete token language can really scale well to that large body of application logic.