Re: Another example for syntactically lightweight closures

["steve donovan" <steve.j.donovan@...>]

On Feb 7, 2008 4:30 AM, David Manura <dm.lua@math2.org> wrote:
> Approaches for syntactically lightweight closures are also discussed in [1].
>
> [1] http://lua-users.org/wiki/ShortAnonymousFunctions
>

I particularly liked the trick where one makes code strings executable
functions. This combines a few ideas in the discussion, and implements
a simple form of caching.

local llookup = {}

getmetatable('').__call = function(s,...)
    local args,body = s:match('%s*|(.-)|%s*(.+)')
    if not args then
        args = ''
        body = s
    end
    s = 'function('..args..') return '..body..' end'
    local fun = llookup[s]
    if not fun then
        print 'compiling'
        local code = "return " .. s
        fun = assert(loadstring(code))()
        llookup[s] = fun
    end
    return fun(...)
end


print(("|x,y| x+y")(2,3))

print( ("|x,y| x+y")(20,30))

function for_n(n,fun,...)
    for i = 1,n do fun(...) end
end

for_n(3,"print(42)")

output:
compiling
5
50
compiling
42
42
42

steve d.