[Swift-devel] iterations on cartesian and inner products

[Ben Clifford]

On Mon, 14 Jan 2008, Mihael Hategan wrote:

> I'm thinking the following might be nice to have:
> 
> X[]
> Y[]
> 
> for (x, y) in (X, Y) {
>   ...
> }
> 
> for (x, y) in (X * Y) {
>   ...
> }

Rather than implementing it in the iteration construct, a Haskelly (and 
more general) way to do this would be something like this:

i) put anonymously-typed tuples into the language

ii) have proper tuple assignment. we already have this sort-of for 
executions, where you can say

(x,y) = foo();

if foo() is a procedure, but not with an expression. So you can't say

(x,y) = @foo()

iii) put the two array product operations into the langauge or library, so 
that you can in an expression construct the desired product

iv) make foreach understand tuple assignments - i.e. given an array of 
pairs (or n-tuples) assign them in sequence to 


I think this would need anonymous tuple types in the language, but I think 
I'd like that anyway - as we move towards unifying foo() and @foo() into a 
single language construct, I think its desirable to be able to figure
out sensibly what is wrong with

  5 + foo()

when foo returns (int a, int b) - i.e. give a type error that (int) is 
incompatible with (int,int).

At present, 5 + foo() is a syntax error because you cannot invoke a 
procedure inside an expression; but that would go away under foo/@foo 
unification.

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