Function Composition

There are two ways to handle basic function composition with pymonad: Compose and Pipe. Both are really just aliases for the Reader monad with names that are a bit more semantically meaningful for the way that they're used, while Reader itself is used slightly differently.

Compose

Compose takes a series of functions and combines them in a feed-forward manner producing a new function.

from pymonad.reader import Compose

def add_1(x):
    return x + 1

def mul_3(x):
    return 3 * x

# Creates a new function that first adds 1, then multiplies by 3, and
# finally converts the result to a string.
new_func = (Compose(add_1)
            .then(mul_3)
            .then(str)
)

print(new_func(2)) # '9'

Pipe

Pipe is similar to Compose but rather than starting with a function it starts by taking in a value and then feeding it through the following functions.

from pymonad.reader import Pipe

def add_1(x):
    return x + 1

def mul_3(x):
    return 3 * x

result = (Pipe(2)
         .then(add_1)
         .then(mul_3)
         .then(str)
         .flush()
)

print(result) # '9'

There are two things to note here.

While Compose results in a new function, Pipe produces a result.
In order to access the result you need to call flush() at the end of the pipe operation.¹

Pipe is the only construct in pymonad with a defined operator: The unary +. The unary + operator typically just returns the value it's given without modifying it in any way. With Pipe it's used to return the value which the pipe produces. In other words, you can use '+' instead of flush(), like so:

result = (+Pipe(2)
         .then(add_1)
         .then(mul_3)
         .then(str)
)

print(result) # '9'

Footnotes:

Since Pipe is based on the Reader monad, without flush() you would actually get a function as a result. That function, however, would ignore it's input and always give the same result. flush() simply calls this function with dummy input and gives you back the result.