Functional programming

Functions are provided to use typical functional programming. It’s actually a kind of iterative programming.

filter() - filters a stream

A stream can be filtered to find values matching a predicate.

./streem find-larger.strm

Values 100, 200, and 300 are filtered out because they are less than or equal to 300, and therefore don’t match the predicate.

array = [100, 200, 300, 400, 500]
predicate = filter { x -> x > 300 }
array | predicate | stdout

# Output:
# 400
# 500

take() - take some elements from a stream

Instantiates a new stream that consists of some elements from an input stream. The number of elements is passed as an argument.

random = [0.978470, 0.396758, 0.241088, 0.543190, 0.598714, 0.451562, 0.944703]
random | take(3) | stdout

# Output:
# 0.97847
# 0.396758
# 0.241088

flatmap - flattens the input stream

You can make an array out of a matrix (array of arrays) by calling ‘flatmap()’. Just pass the identity function as its argument, i.e. the ‘{ x -> x }’ lambda expression. Note that the ‘x’ variable in the expression is by itself a stream.

matrix = [[1, 2], [3, 4]]
identity = { x -> x }
matrix | flatmap(identity) | stdout

# Output:
# 1
# 2
# 3
# 4

Sequence of numbers

To produce a sequence of numbers (1, 2, 3, …, n) for a given n, call the ‘seq’ function.

./counter.strm

seq(3) | stdout

# Output:
# 1
# 2
# 3

reduce - reduce a stream to an object

To reduce a stream, pass the reduction function as the ‘reduce’ function argument.

./streem sum-numbers.strm

In this example, the sequence of numbers from 1 to 50 is summed. It can be done using a mathematical theorem, and the result is 1275. The Streem function that does it is ‘{ x -> x * (x + 1) / 2 }’.

# Output: 1275

seq(50) | reduce { x, y -> x + y } | stdout

each - do for each element in a stream

For each element in an input stream, you can execute a function. There’s no output stream required.

./streem five.strm

This checks whether the passed argument is 3. It prints “true” if yes. Otherwise, it prints “false”.

# Input:
# 2
# 2
# 2
# 3
# 2

stdin | each { x -> puts(number(x) == 3) }

# Output:
# false
# false
# false
# true
# false