[clean-list] Arrays: type annotation and pattern matching

[terrence.x.brannon at jpmchase.com]

I am confused on the type annotation for this function:

CopyFirst :: Int *(a e) -> *(a e) | Array a e
CopyFirst j a=:{[0]=a0} = {a & [j] = a0}

It looks like the function CopyFirst takes two arguments. The first is an 
integer. The second argument confuses me. The asterisk in front of (a e) 
means some data type which receives destructive updates.
But the syntax (a e) was not previously introduced. 

And then the type restriction confuses me...

| Array a e

seems to say 'provided a and e are of type Array' ?

And then the actual code of the function confuses me... actually I think I 
got it

First:   CopyFirst j a=:{[0]=a0}

the function takes 2 arguments, j (an integer) and then a, an array, but 
it also sets a0 to the first element of the array a... of course this 
looks odd. Why is a0 on the RHS instead of LHS?

Then: the actual function body starts ... {a & [j] = a0}  is saying 'take 
a and update the jth position with the value a0'
but the problem is a0 is never defined.

So, to summarize:

1 - the syntax *(a e) is used but not previously defined
2 - the type restriction | Array a e is a bit confusing
3 - it seems odd that a0 is on the RHS of {[0] = a0} instead of the LHS



--
Terrence Brannon - SID W049945
1111 Polaris Pkwy, OH1-1141, Columbus, OH 43240 
818-359-0893 (cell)
614-213-3426 (fax)





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