Re: Numeric for loops...
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- Subject: Re: Numeric for loops...
- From: steve donovan <steve.j.donovan@...>
- Date: 2010年5月31日 12:39:32 +0200
On Sat, May 29, 2010 at 9:21 AM, duck <duck@roaming.ath.cx> wrote:
> The numeric 'for' loop is so simple, so quick, so easy to explain, so
> commonplace in programming (and so commonly provided in programming
> languages), and so jolly useful that I would like to suggest that we all
> agree to stop calling for its removal :-)
An excellent sentiment!
One can use macros to get some sugar over the basic for-loop.
For instance, this macro expands foreach x in t do into for _i=1,#t do
local x = t[_i]. It goes further; if the table parameter is an
expression then it synthesizes an enclosing block and creates a local
alias for that table expression. So slightly different code is
generated for these two cases:
t = {'one','two','three','four','five'}
foreach val in t do print(val) end
foreach k in {10,20,30} do print(k) end
Macros are often considered an 'unclean' solution to syntactical
clarity issues, but they can be very useful to _try out_ a proposed
syntax.
--- foreach.lua
macro.define ('foreach',nil,
function (ls)
-- getting
local get = ls.getter
local _,t1,name = get()
local _,t2 = get()
if t2 ~= 'in' or t1 ~= '<name>' then macro.error("expecting
'in' after single loop variable") end
-- grab everything up to do
local args = macro.grab_parameters('do','')
if not args then macro.error("expecting 'do'") end
args = args[1]
-- putting
local subst,put = macro.subst_putter()
local function putn (n) put('<name>',n) end
local tbl
if #args > 2 then -- the table parameter is an expression;
factor out as an upvalue
tbl = '_t'
put 'do'; put 'local'; putn(tbl); put '='; put(args)
else
tbl = args[2]
end
put 'for'; putn '_i'; put '='; put('<number>',1); put ','; put
'#'; putn(tbl); put 'do';
put 'local'; putn (name); put '='; putn(tbl); put '['; putn
'_i'; put ']'
if #args > 2 then -- the for-loop is enclosed in a block
macro.set_end_scanner ('end end',-1)
end
return subst
end
)
-----
steve d.