gp/pari version of the "Map" datatype and "reduce" function

[Richard Heylen <richardheylen@gmail.com>]

When writing gp/pari programs I often have a function that produces
lots of different discrete values and I want to record some data about
these values. For example, I might want to accumulate sets of input
values that result in the different output values. The key feature is
that I don't really know what the output values are before I start
running the function and they can't conveniently be converted into
integers to be used as an index into an array.
In other languages this is often addressed by providing a data type
which can take an arbitrary value as a key and lets you set or return
an arbitrary value associated with that key. As a bonus, it might be
useful to be able to interate over the data in the order in which the
keys were first used.
I can achieve something similar at the moment using a combination of a
Set to hold the keys and a Vector for the values, using setsearch to
find the index for the vector but it's not nice.

Also, pari/gp has functions "apply" and "select" which implement
common functional programming ideas. A key one which seems to be
missing is "reduce" (or "fold") which takes a vector, a function and
an initial value  and applies the function repeatedly to the value and
the next element of the vector, using the return value as the value
for the next iteration. For example reduce((a,b)->a+b,v,0) would
return the sum of the elements of v.

Any help would be appreciated!

Richard