Multiple assignment

We should also expand this to include list indexing for the lvalue. e.g.

list[a, b, c] = someListExpr

would expand to

t = someListExpr
list[a] = t[0]
list[b] = t[1]
list[c] = t[2]

The comma operator would also conflict with tuples, which are also planned.

See also the GEP as it currently stands: http://docs.codehaus.org/display/GROOVY/Multiple+Assignment+Proposal

since there was low feedback on this the only form that is supported atm is

a,b = [1,2]

meaning no declaration and the number of arguments must match

Just as a comment for future Jira searchers. This does allow Groovy to do the often discussed swap two variables without a temp variable! E.g.:

def a = 1
def b = 2
[a, b] = [b, a]
println a // => 2
println b // => 1

This code doesn't work:
<code>
def a = 1
def b = 2
[a, b] = [b, a]
println a // => 2
println b // => 1
</code>

Environment:

• Groovy Version: 1.6.0 JVM: 1.6.0_12

Error:
<error>
class org.codehaus.groovy.ast.expr.ListExpression, with its value '[a, b]', is a bad expression as the left hand side of an assignment operator at line: 4 column: 8. File: /home/ms/work/groovy/test04.groovy @ line 4, column 8.
[a, b] = [b, a]
^

1 error

</error>

per http://jira.codehaus.org/browse/GROOVY-1543?focusedCommentId=150396&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#action_150396

it's now:

def a = 1
def b = 2
(a,b) = [b, a]

to be working.

However, I think

def (head, tail) = [1,2,3,4]
would work as in the issue descrption:

head should be 1
and tail should be [2,3,4]

Current behavior doesn't have special head/tail treatment.

(one, two) = "The quick brown fox".tokenize()
assert one == 'The'
assert two == 'quick'

in discussion was to mark an element to take the surplus parts as list.. for example like:

(head, *tail) = [1,2,3,4]
assert head == 1
assert tail = [2,3,4]

(*head, tail) = [1,2,3,4]
assert head == [1,2,3]
assert tail = 4

(head, *middle, end) = [1,2,3,4]
assert head == 1
assert middle = [2,3]
assert end == 4

I thought about doing maybe even something like this:

(*a,3,*b) = [1,2,3,4]
assert a == [1,2]
assert b == [4]

But that is just an idea, not really discussed and I need use cases for these first. And normally you don't look for use cases because you want to have a feature, instead you have a problem and think of a feature to solve that use case.

Basically you can have these cases

(a,b) = [1,2]     // b== 2, or b== [2] ?
(a,b) = [1]        // exception, or b==[], or b==null?
(a,b) = [1,2,3]  // b==2, or b==[2,3], or exception?

I decided for b==2, exception, b==2. But any other version seems to be equally reasonable. With a fix head/tail logic for example I would expect b==[2], b==[], b==[2,3]. But is this really multiple assignment? It looks more like a combined car/cdr operation, which does not exactly fit into Groovy atm.

If you are interested in getting head tail logic into groovy, then I suggest you start a thread on the user list for it.

So what's the final result, what's been provided for "multiple assignment"?

My last comment and Jochen's last example briefly show what is currently possible or check out the following test:
http://svn.groovy.codehaus.org/browse/~raw,r=13691/groovy/trunk/groovy/groovy-core/src/test/gls/statements/MultipleAssignmentDeclarationTest.groovy

Or this test:
http://svn.groovy.codehaus.org/browse/~raw,r=13691/groovy/trunk/groovy/groovy-core/src/test/gls/statements/MultipleAssignmentTest.groovy

Perfect, thanks.