Pairs are 2-tuples of values that are constructed and matched with the `::`
operator. They can also be matched with a `:` operator, the LHS is an
expression then, the pair will then only match, if the LHS matches the
result of that expression.
Pairs should be useful to do something similar what sum types / tagged
unions do in statically typed languages, e.g. you could write something
like:
some := (symbol) # Somthing that creates a unique value
filter-map := {
_ [] -> []
f [x ~xs] ->
{
some:y -> [y ~(filter-map f xs)]
nil -> filter-map f xs
} (f x)
}
filter-map {
x?even -> some :: (* x 10)
_ -> nil
} some-list
You can now set keys in dictionaries to a value. If a key in a key path is
missing, we automatically create an empty dictionary. Otherwise setting
deeply nested keys becomes annoying.
