Haskell: How to organize a group of functions that all take the same arguments

Question

I am writing a program with several functions that take the same arguments. Here is a somewhat contrived example for simplicity:

buildPhotoFileName time word stamp = show word ++ "-" ++ show time ++ show stamp
buildAudioFileName time word = show word ++ "-" ++ show time ++ ".mp3"
buildDirectoryName time word = show word ++ "_" ++ show time

Say I am looping over a resource from IO to get the time and word parameters at runtime. In this loop, I need to join the results of the above functions for further processing so I do this:

let photo = buildPhotoFileName time word stamp
    audio = buildAudioFileName time word
    dir   = buildDirectoryName time word
in ....

This seems like a violation of "Don't Repeat Yourself" principle. If down the road I find I would like to change word to a function taking word, I might make a new binding at the beginning of let expression like so:

let wrd   = processWord word
    photo = buildPhotoFileName time wrd stamp
    audio = buildAudioFileName time wrd
    dir   = buildDirectoryName time wrd
in ....

and would have to change each time I wrote word to wrd, leading to bugs if I remember to change some function calls, but not the others.

In OOP, I would solve this by putting the above functions in a class whose constructor would take time and word as arguments. The instantiated object would essentially be the three functions curried to time and word. If I wanted to then make sure that the functions receive processWord word instead of word as an "argument", I could call processWord in the constructor.

What is a better way to do this that would be more suited to Functional Programming and Haskell?

Answer 1

Since you say you're ready to create an OO-wrapper-class just for that, I assume you're open to changing your functions. Following is a function producting a tuple of all three results you wanted:

buildFileNames time word stamp = 
  ( show word ++ "-" ++ show time ++ show stamp,
    show word ++ "-" ++ show time ++ ".mp3",
    show word ++ "_" ++ show time )

You'll be able to use it like so:

let wrd   = processWord word
    (photo, audio, dir) = buildFileNames time wrd stamp
    in ....

And if you don't need any of the results, you can just skip them like so:

let wrd   = processWord word
    (_, audio, _) = buildFileNames time wrd stamp
    in ....

It's worth noting that you don't have to worry about Haskell wasting resources on computing values you don't use, since it's lazy.