for

The keyword for denotes iteration over lists or iterators

One common example would be to iterate over number ranges:

for 1..10 : print it // excluding 10
for 1...10 : print it // including 10
for 1 to 10 : print it // including 10
for 1 upto 10 : print it // excluding 10

Following swift, and the possibile confusion of developers, we might is_declared warnings or ask if they know what they are doing.

for … in

While the in operator may return lists compatible with for, it may also return other objects. In this case in binds with for to iterate over all such items:

3 in [1,2,3,4] // yes; the position comes from [1,2,3,4].index_of(3) == 3
for x in [1,2,3,4] : print x // different situation

Same with of from

Ruby's form works too, with a hint naming the braces: for x in 0..3; print x; end is for x in 0..3 { print x }.

for TYPE in …

Interesting: structurally type-matching for condition

for friend in [foe1, friend1, foe2, friend2]: print it 

ONLY iterates over those elements of specified type (here:friend)!

This idea can be extended:

for LAMBDA CONDITION in …

for (it>2) in [1,2,3,4] : print it 

for PATTERN in …

for (even number) in [1,2,3,4] : print it 

Runs today (warp)

The end of a range is read once, before the first round, as in Python: n = 3; for i in 0..n { n = 10 } runs three rounds.

In for x in xs the variable is also it; a block or lambda inside the body keeps its own it. print name in text prints each item, as for name in text: print it:

for word in ["a", "b"]: print it

print chars in "hey"