Begin End Proposal

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This is a formal proposal I wrote up. Let me know what you think, but please don't modify the text of the proposal itself. -- LeafStorm

This is not a reasonable policy for a public wiki page. Please change it or move the content to a read-only location on the web. --JohnBelmonte

Abstract

This is a proposal for the addition of 'begin/end' syntax for anonymous, zero-argument functions, aka "procs". For example, this:

begin
  print "Foo!"
end

would be effectively transformed into:

function ()
  print "Foo!"
end

This could be added in LuaFiveTwo, or possibly in Six (though admittedly Six is a fairly faraway time frame for a relatively minor feature like this.)

Syntax Details

The precise modifications to the syntax could be expressed as:

proc ::= 'begin' block 'end'

exp ::= nil | false | true | Number | String | `... | function | proc | 
    prefixexp | tableconstructor | exp binop exp | unop exp 

args ::= '(' [explist] ')' | tableconstructor | String | proc | function

(This adds to the grammar expressed in the Lua 5.1 reference manual.) The 'begin' keyword would be added to the list of keywords, and as such would become unavailable as an identifier.

When a 'proc' is encountered, a function is created with zero arguments and the block as the body. Any arguments passed to said function are passed as variable arguments, accessible by the ... expression (and the arg table if that functionality remains when this proposal is implemented).

The other notable modification to the syntax is the ability to pass either a proc or an anonymous function straight to the function without parens, like is currently done with table constructors and the like.

Rationale

The current syntax which could generate something like a proc is verbose and unwieldy.

function ()
  do_stuff()
end

The new syntax would be much easier to read, easier to type, and easier on people using Lua as a configuration language or DSL. The ability to pass functions and procs to function calls without parens would serve a similar purpose. It would require minimal modifications to the Lua core, and code using procs would treat them no differently than normal functions.

Examples

This is an example I thought of. (Of course, there are many other places this could be used.)

HTML Generation

doc = html begin    -- 'html' is a function in the local namespace
  head begin        -- 'head' is automatically generated by the environment
    title "Hello, world!"
  end
  body begin
    h1 "Hello, world!"
  end
end

Backwards-Compatibility Concerns

At the bytecode level, this would be completely backwards compatible - procs could generate the same bytecode as the function () ... end syntax.

Syntactically, the changes would be minimal. The only syntax added that could cause problems with existing code is the addition of begin as a keyword. (The other syntax additions were not even remotely possible before, and should not cause issues with current code.) However, begin isn't a very good identifier - there is precedent in other languages for its use as a keyword, and often start is a better fit in situations where begin could be used.

Alternatives Considered

I originally thought of the use of start as a keyword. However, there are major problems with this. First, it is used as an identifier much more often than begin. Second, start has never been used in this setting to my knowledge, but begin has in many languages. Finally, begin sounds better and takes the same number of characters to type.

Another way to approach this would be to use a delimiter. Using curly braces is not practical, as they are already used by tables - and it would be much more difficult to distinguish tables from code blocks in this case. Square brackets have similar problems, in that it would be too easy to have them confused with long-string delimiters. And angle brackets have never been used this way before, and could be misconstrued as a binary comparison operator. Parens are just ridiculous. And using words is more of the Lua way.

Publication Notes

This proposal was written by LeafStorm.

Lua code using syntax introduced in this proposal is written in normal monospace format to avoid confusing the syntax highlighter.

This is the end of the actual proposal.

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Last edited October 3, 2009 4:14 pm GMT (diff)