Definitions can be made local to an expression by enclosing them in a `let within' clause.
primes =
let
factors(n) = < m | m <- <2..n-1>, n%m == 0 >
is_prime(n) = null(factors(n))
within < n | n <- <2..>, is_prime(n) >
Local definitions are mutually recursive, just like top-level definitions. Not all definitions can be scoped in this way: channel and datatype definitions are only permitted at the top-level. Transparent definitions can be localised, and this can be used to import FDR2's compression operations on a selective basis. For example,
my_compress(p) =
let
transparent normal, diamond
within normal(diamond(p))
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