In a recent homework assignment I ended up calling my functions in an ugly way uglyReceipt(cashParser(cashInput())) the program itself worked perfectly but I still felt like I was doing something wrong.

Is calling functions like this bad practice and if so: What should I be doing instead?

  • Possible duplicate of Nested functions; allow or not? – gnat Apr 17 '16 at 11:08
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    Are those nested functions, or is that a nest of calls to functions ? If the latter OP may have (re-)invented functional programming. – High Performance Mark Apr 17 '16 at 11:11
  • What makes you think this would be a bad practice? – Ixrec Apr 17 '16 at 13:34
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    @gnat: That question is completely unrelated. That question is about lexically nested function definitions, this question is about passing the result of a function call as an argument to another function call. – Jörg W Mittag Apr 17 '16 at 14:07
  • I misread - thanks for pointing @JörgWMittag (retracted vote) – gnat Apr 17 '16 at 14:20

This really depends on how much nesting you use. After all, you are allowed to use function results directly in expressions to improve readability. Both, code that does not use nested expressions (like assembler code), and code that uses too much nested expressions is hard to read. Good code tries to strike a balance in between the extremes.

So lets look at some examples. The one you gave in your question seems quite legit to me, so nothing to worry here. However, a line like

foo(bar(baz(moo, fab), bim(bam(ext, rel, woot, baz(moo, fab)), noob), bom, zak(bif)));

would definitely not be tolerable. Likewise, code like

double xsquare = x*x;
double ysquare = y*y;
double zsquare = z*z;
double xysquare = xsquare + ysquare;
double xyzsquare = xysquare + zsquare;
double length = sqrt(xyzsquare);

would not be very readable as well. sqrt(x*x + y*y + z*z) is much easier to understand, even though it combines a total of six different operation in one expression.

My advice is to pay attention to what expressions you can still parse in your head easily. The moment you need to take a second look to grasp what a single expression does, it's time to introduce an additional variable.

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I think whether it's good or bad depends a lot on context. The main reason it might be considered bad is that it (arguably) makes the code harder to read and debug. This is especially true when you are first learning to program. As your coding skills (and code) gets more advanced, there are times when this is acceptable.

For example, consider an error message like this:

line 1492: missing argument "foo"

How do you know if the missing argument is to cashInput, cashParser, or uglyReceipt? Depending on the language the error message might tell you, but it might not.

If were were to break those function calls apart, and the error message still pointed you to line 1492, you would know instantly where the problem lies:

1491: input = cashInput()
1492: parsed_value = cashParser(input)
1493: receipt = uglyReceipt(parsed_value)

With the steps broken out separately, it is much easier to debug since it is possible to set a breakpoint at any step, and you can easily inject values by changing the value of the local variables.

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The concept underlying your question is so important I feel it needs another answer rather than just a comment (as I had started to do).

The other 3 answers thus far provide some useful points of consideration on whether a given situation merits using what you call "nested function calls". But perhaps a more important point is hidden in the comments under your question: in case you missed the subtlety in what those erudite folks are suggesting, Carl, you have discovered for yourself the topic actually called functional programming. If you have never seen the term, you might not have thought it was really a "thing" in @HighPerformanceMark's comment.

But indeed it is! Functional programming has been written about for decades, since John Hughes' seminal paper Why Functional Programming Matters. There are some languages that are functional languages (i.e. they only let you write in a functional programming style), languages like Erlang, Lisp, OCaml, or Haskell. But there are many more languages that are hybrid imperative/functional languages. That is, they are traditionally imperative languages but offer some support for functional programming as well, including Perl, C++, Java, C#, and many more. Wikipedia's entry on functional programming provides a nice section showing a comparison of functional style vs. imperative style for a number of languages.

There is much to say on the differences between imperative and functional styles, but the key starting point is that with functional programming, functions or methods have no side effects, making it in general easier to both understand and debug programs.

For further reading, you might also take a look at Reginald Braithwaite's Why "Why Functional Programming Matters" Matters and another interesting post here on SO, Why functional languages?

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It is absolutely not a bad practice in general. Functions call accept values and one way of producing a value is by calling another function.

When I see a variable being defined, like:

parsed_value = cashParser(input)

... I have to consider that parsed_value might be used more than once and I'll probably have to check if this is true or not (what if my change breaks something elsewhere?). When you start adding more variables, it can become more complex for the reader to keep track of all of them and how they are related. So I am relieved when I see:

receipt = uglyReceipt(cashParser(input))

... because the intermediate value's scope/lifetime is obvious. Now, as always, splitting a long expression into separate statements might help, especially if a variable name can give more precision about the purpose of a value:

user_name = input()
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