Here's my solution to the problems I posed.
Don't peek if you are trying to do it yourself.
rebol []
comment1: {
Imperative programs that mimic basic functional operations
mostly through an intense amount of copying.
}
prepend: func [item blk] [
result: make block! (length? blk) + 1
insert result item
insert next result blk
return result
]
ndappend: func [b1 b2] [
result: make block! (length? b1) + (length? b2)
insert insert result b1 b2
return result
]
filter: func [f blk][
result: make block! (length? blk)
foreach element blk [
if f element [result: insert result element]
]
return head result
]
comment2: {
No more imperatives. All the following functions are
`pure' functions.
}
divisible-by?: func [num divisor] [zero? num // divisor]
prime-seive: func [blk] [
any [all [empty? blk blk]
prepend
first blk
prime-seive
filter func [x] [not divisible-by? x first blk]
next blk]
]
accumulate: func [f init block] [
any [all [empty? block init]
f first block accumulate :f init next block]
]
head-elements: func [n block] [
any [all [zero? n []]
prepend first block head-elements n - 1 next block]
]
enumerate-interval: func [start end] [
any [all [start = end []]
prepend start enumerate-interval start + 1 end]
]
enumerate-tree: func [tree] [
any [all [not block? tree reduce [tree]]
all [empty? tree []]
ndappend
enumerate-tree first tree
enumerate-tree next tree]
]
comment3: {
Because the programs are purely functional, we can mix and match
the elements like so many legos.
}
;; Task 1 compute the product of the first 10 odd numbers
task1: [accumulate :* 1
head-elements 10
filter :odd?
enumerate-interval 1 20]
;; Task 2 compute the sum of all the primes under 100
task2: [accumulate :+ 0
prime-seive
enumerate-interval 2 100]
;; Task 3 given the following nested structure, return a block
;; of the strings present in the structure.
task3-input: [[[3 "Hello"] [6 "world"] [7 "from" 21 ["REBOL"]]]
[[["hi"] ["this" ['aword] "is"] "a"] "test"]]
task3: [accumulate :prepend []
filter :string?
enumerate-tree
task3-input]
;; Just call do task1 or do task2 or do task3 to see it work.
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