fmap(), "inverse" of Python map() function

By Vasudev Ram

fmap() is a function I created, which is a kind of inverse of the built-in Python map() function. It has probably been discovered/created by many others before (though they may have called it by different names), but I thought of it just now, based on some code I wrote recently (*), so blogging about it.

The comments in the source code below describe what fmap does. It is straightforward.

(*) That "recent code" refers to this snippet:

result = item
for processor in self._processors:
    result = processor(result)

from my blog post about the release of PipeController v0.1; that snippet basically does the same thing as fmap(), but the snippet is specific to PipeController, whereas fmap() was extracted from that, and generalized to be reusable in other programs.

fmap.py source code:

(You can also get the fmap.py code from a pastebin here, since Blogger sometimes doesn't render inline code too well (tabs show up as one space, for example.)

# fmap.py

# Author: Vasudev Ram - http://www.dancingbison.com

# fmap() is a Python function which is a kind of inverse of the 
# built-in Python map() function.
# The map() function is documented in the Python interpreter as
# follows:

"""
>>> print map.__doc__
map(function, sequence[, sequence, ...]) -> list

Return a list of the results of applying the function to the items of
the argument sequence(s).  If more than one sequence is given, the
function is called with an argument list consisting of the corresponding
item of each sequence, substituting None for missing values when not all
sequences have the same length.  If the function is None, return a list of
the items of the sequence (or a list of tuples if more than one sequence).
"""

# The fmap() function does the inverse, in a sense.
# It returns the result of applying a list of functions to a 
# given argument.
# TODO: Later extend the function to also work on a sequence of 
# arguments like map() does.

import string

def fmap(function_list, argument):
 result = argument
 for function in function_list:
  #print "calling " + function.__name__ + "(" + repr(result) + ")"
  result = function(result)
 return result

def times_two(arg):
 return arg * 2

def square(arg):
 return arg * arg

def upcase(s):
 return string.upper(s)

def delspace(s):
 return string.replace(s, ' ', '')

def main():

 print

 function_list = [ times_two, square ]
 for argument in range(5):
  fmap_result = fmap(function_list, argument)
  print "argument:", argument, ": fmap result:", fmap_result

 print

 function_list = [ upcase, delspace ]
 for argument in [ "the quick brown fox", "the lazy dog" ]:
  fmap_result = fmap(function_list, argument)
  print "argument:", argument, ": fmap result:", fmap_result

if __name__ == "__main__":
 main()

# EOF: fmap.py

Output of running a test program for fmap():

$> python fmap.py

argument: 0 : fmap result: 0
argument: 1 : fmap result: 4
argument: 2 : fmap result: 16
argument: 3 : fmap result: 36
argument: 4 : fmap result: 64

argument: the quick brown fox : fmap result: THEQUICKBROWNFOX
argument: the lazy dog : fmap result: THELAZYDOG

UPDATE:

Here are a couple of other interesting posts about functional programming in Python, which I found by doing a Google search for relevant terms:

Dhananjay Nene's two posts on the subject:

Functional Programming With Python - Part 1

Functional Programming With Python – Part 2 - Useful Python Constructs

A StackOverflow thread: Why program functionally in Python?


- Vasudev Ram - Dancing Bison Enterprises

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