Re: Proposal for a new construct in Lua
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- Subject: Re: Proposal for a new construct in Lua
- From: Dirk Laurie <dpl@...>
- Date: 2011年5月16日 23:38:08 +0200
On Mon, May 16, 2011 at 08:47:15PM +0200, Michael Rose wrote:
>
> * Every entry has now an implicit 'break' statement at the end unless
> an 'and' keyword is placed between two entries. E.g.
> jump chr in "0z" do
> "aeiou" do print("vocal") end and
> "a" .. "z" do print("lower letter") end
> "0" .. "9" do print("digit") end and
> do print("no lower letter") end
> end
> works now as expected.
This syntax is much cleaner than the first proposal. I'll feel happier
with a hyphen for range, and included in the string: "aeiou" stays the
same but now "a-z", "0-9". Patterns, sort of, but not the same as that
of the string library. You are matching only one character and don't need
that generality. You could define e.g. "a-z^rs" to mean "a to z but not
r or s". The price paid the pattern-like notation is that '%', '-' and
'^' are now 'magic' characters, to be escaped with '%'.
If you are willing to drop the fall-through, i.e. no "and", and the
duplication (each character appears only once in a key), it would no
longer matter in what order the options are specified, and it would
become possible to implement 'jump' as a function that constructs
a table of functions. No language extension would be necessary:
everything is done by the function.
require "jump"
-- The above example would be coded as
jump { "0-z",
aeiou = 'print("vocal")',
['a-z^aeiou']='print("lower letter")',
['0-9'] = 'print("digit")',
'print("no lower letter")' } [chr] ()
-- But one could also say
JT = jump { "0-z",
aeiou = 'print("vocal")',
['a-z^aeiou']='print("lower letter")',
['0-9'] = 'print("digit")',
'print("no lower letter")' }
-- which allows the jump table to be reused and updated
JT.e() --> vocal
JT['7']() --> digit
JT['A']() --> no lower letter
JT['y']() --> lower letter
JT['ü']() --> error, out of range
JT['ü']= JT.u --> extra case not covered by "a-z"
JT['ü']() --> vocal
So one would need:
1. A system by which a string is interpreted as a pattern defining
one character.
2. A function 'jump' that accepts a table T, with T[1] defining the
range of allowable characters, T[2] if given the default action,
T[pat] the action for characters that satisfy pat; and returns
a table J in which an action is specified for all characters defined
by T[1]. One could even code this function in Lua, but of course
it will be more efficient if done in C.
Dirk