List comprehensions: Difference between revisions

=={{header|Lua}}==

Lua doesn't have list comprehensions built in, but they can be constructed from chained coroutines:

<lang lua>

LC={}

LC.__index = LC

function LC:new(o)

o = o or {}

setmetatable(o, self)

return o

end

function LC:add_iter(func)

local prev_iter = self.iter

self.iter = coroutine.wrap(

(prev_iter == nil) and (function() func{} end)

or (function() for arg in prev_iter do func(arg) end end))

return self

end

function maybe_call(maybe_func, arg)

if type(maybe_func) == "function" then return maybe_func(arg) end

return maybe_func

end

function LC:range(key, first, last)

return self:add_iter(function(arg)

for value=maybe_call(first, arg), maybe_call(last, arg) do

arg[key] = value

coroutine.yield(arg)

end

end)

end

function LC:where(pred)

return self:add_iter(function(arg) if pred(arg) then coroutine.yield(arg) end end)

end

</lang>

We can then define a function to compute Pythagorean triples as follows:

<lang lua>

function get(key)

return (function(arg) return arg[key] end)

end

function is_pythagorean(arg)

return (arg.x^2 + arg.y^2 == arg.z^2)

end

function list_pythagorean_triples(n)

return LC:new():range("x",1,n):range("y",1,get("x")):range("z", get("y"), n):where(is_pythagorean).iter

end

for arg in list_pythagorean_triples(100) do

print(arg.x, arg.y, arg.z)

end

</lang>