Curry: An Integrated Functional Logic Language

since the variables occurring in parameters of the left-hand side are only visible in the corresponding
conditions and right-hand sides. The visibility of other entities can be restricted using let in
expressions or where in defining equations.
An expression of the form let decls in exp introduces a set of local names. The list of local
declarations decls can contain function definitions as well as definitions of constants by pattern
matching. The names introduced in these declarations are visible in the expression exp and the
right-hand sides of the declarations in decls, i.e., the local declarations can be mutually recursive.
For instance, the expression
let a=3*b
b=6
in 4*a
reduces to the value 72.
Auxiliary functions which are only introduced to define another function should often not be
visible outside. Therefore, such functions can be declared in a where-clause added to the righthand
side of the corresponding function definition. This is done in the following definition of a fast
exponentiation where the auxiliary functions even and square are only visible in the right-hand
side of the rule for exp:
exp b n = if n==0
then 1
else if even n then square (exp b (n ‘div‘ 2))
else b * (exp b (n-1))
where even n = n ‘mod‘ 2 == 0
square n = n*n
Similarly to let, where-clauses can contain mutually recursive function definitions as well as definitions
of constants by pattern matching. The names declared in the where-clauses are only visible
in the corresponding conditions and right-hand sides. As a further example, the following Curry
program implements the quicksort algorithm with a function split which splits a list into two lists
containing the smaller and larger elements:
split e [] = ([],[])
split e (x:xs) | e>=x = (x:l,r)
| e