Should ES begin to standardise pragmas?

Pragmas (ignorable source hints to an implementation) have
become fairly wide-spread in Javascript, but their look and
usage differs widely, and with an increasing number of tools
defining their own pragma conventions, it is only a matter of
time before they start stepping on each others toes.

This seems to call for some guidelines from the language
standard (not to define individual pragmas, but to define a
framework and namespacing for using and defining
pragmas). There is a strawman for this

http://wiki.ecmascript.org/doku.php?id=strawman:pragmas

but it doesn't seem to be part of Harmony yet, and it doesn't
mention that pragmas are alread used in the wild, not just in
ES5.

Some examples:

1. ES5 "use strict"
    ignoreable expression statement

2. IE conditional compilation
    @-prefixed, suggested in, but not limited to comments

3. jslint options (partial overlaps with forks like jshint)
    in-comment
    /*jslint
    /*members
    ..

4. google for javascript pragma for more examples
    (sometimes pragmas for embedded javascript in
    applications, sometimes options in comments for
    JS processors)

Apart from standardizing pragma syntax for options, it
would be nice to annotate code with language extensions
in use (rather than language versions): this is similar to
feature- versus browser-version-testing.

There are also some standard pragmas I would like to
see, to warn me when I am using features I do not like
to use, something like

    //OPTIONS: warn-asi
            // trace ASI semicolons, to simplify verification
    //OPTIONS: warn-hoisting
            // warn if bindings are lifted out of their block
    //OPTIONS: warn-non-standard
            // warn if browser-specific features are used

Below I add some references to examples from another
language, Haskell.

Claus

Haskell has pragmas both for declarative optimization
hints and for language extensions

http://www.haskell.org/onlinereport/haskell2010/haskellch12.html#x19-18800012

on top of this, Haskell packages specify the language
extensions they use

http://haskell.org/cabal/release/cabal-latest/doc/users-guide/authors.html#pkg-descr

The currently known language extensions are listed here,
for Cabal (package manager) and GHC (compiler)

http://haskell.org/cabal/release/cabal-latest/doc/API/Cabal/Language-Haskell-Extension.html#t:Extension

http://www.haskell.org/ghc/docs/7.0-latest/html/users_guide/flag-reference.html#id568865

In addition to language pragmas, there is an inofficial standard
for compiler options, eg, GHC understands OPTIONS_GHC
pragmas, which make most of GHC's options available in
source files (those that make sense on a per-module basis):

http://www.haskell.org/ghc/docs/7.0-latest/html/users_guide/pragmas.html

The list of options useable in source pragmas includes many
warning options

http://www.haskell.org/ghc/docs/7.0-latest/html/users_guide/flag-reference.html#id570580

We've talked about this on TC39. We'll probably do something, but exactly what is hard to say at this point. Designing future-proof pragma syntax requires a bit of gazing into the crystal ball...

Dave

On Apr 17, 2011, at 2:13 AM, Claus Reinke wrote:

> Pragmas (ignorable source hints to an implementation) have
> become fairly wide-spread in Javascript, but their look and
> usage differs widely, and with an increasing number of tools
> defining their own pragma conventions, it is only a matter of
> time before they start stepping on each others toes.
> 
> This seems to call for some guidelines from the language
> standard (not to define individual pragmas, but to define a
> framework and namespacing for using and defining
> pragmas). There is a strawman for this
> 
> http://wiki.ecmascript.org/doku.php?id=strawman:pragmas
> 
> but it doesn't seem to be part of Harmony yet, and it doesn't
> mention that pragmas are alread used in the wild, not just in
> ES5.
> 
> Some examples:
> 
> 1. ES5 "use strict"
>   ignoreable expression statement
> 
> 2. IE conditional compilation
>   @-prefixed, suggested in, but not limited to comments
> 
> 3. jslint options (partial overlaps with forks like jshint)
>   in-comment
>   /*jslint
>   /*members
>   ..
> 
> 4. google for javascript pragma for more examples
>   (sometimes pragmas for embedded javascript in
>   applications, sometimes options in comments for
>   JS processors)
> 
> Apart from standardizing pragma syntax for options, it
> would be nice to annotate code with language extensions
> in use (rather than language versions): this is similar to
> feature- versus browser-version-testing.
> 
> There are also some standard pragmas I would like to
> see, to warn me when I am using features I do not like
> to use, something like
> 
>   //OPTIONS: warn-asi
>           // trace ASI semicolons, to simplify verification
>   //OPTIONS: warn-hoisting
>           // warn if bindings are lifted out of their block
>   //OPTIONS: warn-non-standard
>           // warn if browser-specific features are used
> 
> Below I add some references to examples from another
> language, Haskell.
> 
> Claus
> 
> Haskell has pragmas both for declarative optimization
> hints and for language extensions
> 
> http://www.haskell.org/onlinereport/haskell2010/haskellch12.html#x19-18800012
> 
> on top of this, Haskell packages specify the language
> extensions they use
> 
> http://haskell.org/cabal/release/cabal-latest/doc/users-guide/authors.html#pkg-descr
> 
> The currently known language extensions are listed here,
> for Cabal (package manager) and GHC (compiler)
> 
> http://haskell.org/cabal/release/cabal-latest/doc/API/Cabal/Language-Haskell-Extension.html#t:Extension
> 
> http://www.haskell.org/ghc/docs/7.0-latest/html/users_guide/flag-reference.html#id568865
> 
> In addition to language pragmas, there is an inofficial standard
> for compiler options, eg, GHC understands OPTIONS_GHC
> pragmas, which make most of GHC's options available in
> source files (those that make sense on a per-module basis):
> 
> http://www.haskell.org/ghc/docs/7.0-latest/html/users_guide/pragmas.html
> 
> The list of options useable in source pragmas includes many
> warning options
> 
> http://www.haskell.org/ghc/docs/7.0-latest/html/users_guide/flag-reference.html#id570580
> 
> 
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