__type metafield (was: Re: Lua 5.3 work1 Considering math.isinteger or type())

[Philipp Janda <siffiejoe@...>]

Am 22.07.2013 13:58 schröbte John Hind:
> This could also be usefully extended by a new '__type' metatable key. If
> present, its value would be returned as the second return from type(). So
> classes/types created from table or userdata type could easily be set up to
> return an appropriate sub-type.

If you have type-inheritance and/or duck-typing, knowing the exact type 
isn't all that useful. I very much like Dirk Laurie's `has()` 
approach[1] (although I would choose a different implementation). One 
situation where having `__type` is nice is C code directly accessing a 
userdata's memory (-> `luaL_checkudata`). I currently have my own 
implementation of checkudata which uses the `__type` field to create 
error messages like

    bad argument #2 to '__add' (apr_oflags_t expected, got apr_fileattrs_t)

instead of

    bad argument #2 to '__add' (apr_oflags_t expected, got userdata)

This change would also be backwards compatible (unless someone parses 
those error messages). An alternative implementation could have 
`luaL_newmetatable` store a reverse mapping from metatable to type name 
for `luaL_checkudata` to use in error messages ...

  [1]: http://permalink.gmane.org/gmane.comp.lang.lua.general/99873

Philipp