range expression

[Martin Hurton]

Hi,

The length of nil slice, map, and channel are 0.
What's the expected behaviour of loop when the range expression
evaluates to nil?

- Martin

[Steven]

From the specification:

A "for" statement with a "range" clause iterates through all entries of an array, slice, string or map, or values received on a channel. For each entry it assigns iteration values to corresponding iteration variables and then executes the block.

A nil slice, map, or channel have length zero, meaning they have no entries to iterate over. Therefore, the expected behaviour is that there will be zero iterations, since iteration occurs "for each entry". 

[Rob 'Commander' Pike]

I'm not certain this is well specified. That should be addressed.

They'll all execute zero times, except for the channel case, which
will panic ("range over a nil channel"). Outside of s special case in
select, with specific purpose, nil channels are never helpful,
although you can ask their length. This should be clarified in the
spec, one way or the other.

-rob

[Brad Fitzpatrick]

Filed http://code.google.com/p/go/issues/detail?id=1713