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Am 14.01.2021 um 20:30 schrieb Roberto Ierusalimschy:
Using Lua 5.3.

Code:

return { 1 } [1]

Result:

<eof> expected near '['

Code:

return ( { 1 } ) [1]

Result:

1

The question is, why is the first version not accepted? I am not
asking for an explanation why the details are such and such, but
rather for a rationale, if there is one, that a table "literal"
(constructor) should not appear in some expressions unless it is
wrapped in parentheses.

For regularity, what can come before [exp] is the same stuff that
can come before (explist), forming function calls. Allowing {1}[1]
would also allow {1}(1), which being a function call could be
used as a statement:

  a = b
  {1}(1)     -- "calling" table {1}

That would create another ambiguity in the grammar, similar to the
(already present) ambiguity with parentheses:

  a = b
  (print)(1)     -- calling print or calling b?

We chose to reduce ambiguities.

Note too that all other "literals" also cannot be used in that position:

  4[1];  "hello"[1];  true[1];  nil[1]

But, in the end, it all boils down to taste.

-- Roberto

I made a patch for Lua 5.2 to make those brackets optional. To be backwards compatible, I excluded that call operator.

You find that patch here.

http://lua-users.org/wiki/ThomasJericke

My basic idea back than was to have type notation similar to UML notation:

163764 : uint32 would than create a uint32 userdate that stores 16376, this by registering all conversion function to the metatable of the type number. I never continued the approach as I don't think that breaking compatibility to vanilla Lua is worth the benefit of an arguably nicer syntax.

--

Thomas