Re: REPL reader of multi-language

[Mike Gerwitz <mikegerwitz@gnu.org>]

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On Fri, May 30, 2014 at 04:25:40PM +0800, Nala Ginrut wrote:
> It's not a problem for Ecmascript, because its statement has to be ended
> with semicolon. So it's easier to detect the minimum compiling unit in
> REPL. 

ECMAScript uses semicolon insertion---semicolons are optional, but this can
lead to ambiguities. For example:

  function foo() {
    return
      { foo: "bar" }
  }

`foo` will return undefined, not {foo:"bar}, because it inserts a semi-colon
after `return`; the way to get around this is to put the beginning curly
brace of the object literal on the same line as `return`, in which case the
expression would be completed by the closing curly brace of the object
literal, semicolon optional.

If you take a look at Node.js' REPL, you can see how it handles line
continuations.

So this problem would exist for more than just Lua. But is it actually a
problem? Ctrl+D, if needed to end an expression, would represent EOF; I
don't know if this is valid in Lua, but in ES, if an EOF (or semicolon) is
encountered during the parsing of an expression that requires more input,
then it is a syntax error---the compiler and REPL would handle it in the
same way, but the REPL would just display a PS2-style prompt (which readline
seems to do as "..." right in Guile 2.0).

It could be my ignorance of Lua. :)

-- 
Mike Gerwitz
Free Software Hacker | GNU Maintainer
http://mikegerwitz.com
FSF Member #5804 | GPG Key ID: 0x8EE30EAB

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