Curry: An Integrated Functional Logic Language

A.4 Functional Logic Programming
This is the same example as in the previous section. However, we use here a functional logic
programming style which is more readable but provides the same goal solving capabilities. The
basic functions are husband and mother which express the functional dependencies between the
different persons. Note that the derived function grandfather is a non-deterministic function
which yields all grandfathers for a given person.
data Person = Christine | Maria | Monica | Alice | Susan |
Antony | Bill | John | Frank | Peter | Andrew
-- Two basic functional dependencies:
husband :: Person -> Person
husband Christine = Antony
husband Maria = Bill
husband Monica = John
husband Alice = Frank
mother :: Person -> Person
mother John = Christine
mother Alice = Christine
mother Frank = Maria
mother Susan = Monica
mother Peter = Monica
mother Andrew = Alice
-- and here are the deduced functions and relationships:
father :: Person -> Person
father c = husband (mother c)
grandfather :: Person -> Person
grandfather g = father (father g)
grandfather g = father (mother g)
Expressions and their evaluated results:
father Child =:= john ❀ {Child=susan} | {Child=peter}
grandfather c ❀
{c=Susan} Antony | {c=Peter} Antony | {c=Andrew} Bill | {c=Andrew} Antony
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