Abstractions for associative lists

The following examples show how a list of keys and a corresponding list of values can be combined into one associative list of pairs using the abstraction `map2', an extension of `map'.

To display an associative list using symbols as keys a new function is required, because `ldisp!' can handle only Church Numerals. We call this new function `aldisp!'.

To search for keys in the list we need another lambda abstraction, which we call `assoc'.

This example uses the following extensions of A++ in its original version:

• defnition of symbols using the `single-quote'-character,
• use of the primitive function `equalx' for comparing general objects.
• use of the primitive function `print' to display the results.