Hackle's blog
between the abstractions we want and the abstractions we get.

Dependency hell? Not if we use functions! For library authors

Dependency hell, a typical example of accidental complexity in software engineering, has annoyingly and unfortunately become some sort of "advanced problem". Solutions mostly focus on the consumers' side, varying from overly-specific, convoluted to hacky. A truly depressing state of affairs.

However, if we look further up the chain at when a library (or package) is designed and created, quite a lot of this annoyance can be solved trivially, and the solution is only so glaringly available and laughably simple. You have guessed, more often than not, "function" is the answer, again!

an example

Say I made a lovely server side application A, which uses library B and library C. Along comes the point of interest: A, B and C all use a library D, but in different versions.

If your package manager is not NPM who allows transitive dependencies to be hidden behind the direct dependency, but something like .NET that only allows a single version of D in the output, congratulations, you are in dependency hell.

what's the REAL dependency

The question to ask, is what is the REAL dependency? Or, what's the KEY functionality used by A, B and C?

More often than not, the answer is simpler than expected: if D is used so much, it is possibly some sort of cross-cutting concern. Think about logging, error handling, HTTP call, or as an example, serialisation, which can be boiled down to a simple function.

// no generics here as that on a lone Func value is actually hard
Func<object, string> Serialize;

Knowing that, the authors of library B and C can remove the hard dependency on library D as a serialiser, instead they can ask their consumers to pass in a serialize function of their choice. Here is what library B can look like,

class LibBConfig 
    // why not a method? read on!
    Func<object, string> Serialize;

class LibB 
    public LibB(LibBConfig config)
        this._config = config;

    void SendHttpRequest()
        var body = this._config.Serialize(dto);
        // ... sends request with body via HTTP

Now application A uses library B as below,

var libD = new LibD();
var libBConfig = new LibBConfig { 
    Serialize = obj => libD.Serialize(obj)  // or more directly: Serialize = libD.Serialize
var libB = new LibB(libBConfig);

Same goes for library C. Let me do a copy-paste.

var libCConfig = new LibCConfig { 
    Serialize = obj => libD.Serialize(obj)
var libC = new LibC(libBConfig);

What has happened here? Surprisingly, LibD is only ever used in application A; library B and C each receives Serialize through its OWN configuration types, and are both free of direct dependency on library D. Dependency hell is no more!

This is it, this is the trick! Pass dependencies in as function values.

is it first class?

What may underlie the difference between using a library as a dependency, and passing in function values, is libraries are not first-class in programming. One cannot write an if/else statement to decide if library D should be downloaded for the compilation of library B; it's all done quite unambiguously with a package manager. For example, either we run nuget install LibD or not; there is no other way.

On the other hand, Functions as values are first-class, which means they can be passed around and manipulated much like other types of values. In other examples, we can swap out any implementation of Serialize, as long as it suits the type required by library B or library C. So application A is free to choose whatever serialiser library there is.

interface? No thanks

True believers of Object-Oriented programming will demand an ILibBConfig here as below.

interface ILibBConfig
    string Serialize(object o);

So application A can implement it with a class LibBConfig, to be passed in for new LibB(instance of LibBConfig). This accomplishes the same effect, and may look more "proper", but if we zoom in a little, there is an important difference between interfaces and function values (this is tautology - functions are usually values these days).

An interface like ILibBConfig is closed (especially in languages with a nominal type system). It must be implemented; once a implementation is done, it's hard to swap out one Serialize with another; indeed, one can use "design patterns" such as strategies or decorators, but they are fixes for the problem of closedness, and are closed themselves.

On the other hand, functions are less closed. Let's take another look at LibBConfig.

class LibBConfig 
    Func<object, string> Serialize;

It simply wraps around Serialize, a field of type Func<object, string>. ANY function of this type can be assigned to Serialize, no matter what class or interface it lives in. That's why we could configure to use library D for library B and library C; in another word, Serialize is OPEN. If library B and library C both require Func<object, string>, there is no need to implement two interfaces; we simply pass along LibD.Serialize.

Are interfaces first-class? From the angle of "implementing" an interface, usually, a class cannot conditionally implement an interface. So NO is most likely the answer.


If you are authoring a library, either for public or private use, think about library / package dependencies.

Firstly, there should be rigour in deciding what should be considered a dependency? Is it foundational to the functionality? Or is it non-essential, but something the library uses and can be swapped out for something else?

Understanding once a dependency is introduced, the constraints can be carried on to the users, and can further constrain their options and make their lives hard.

Think twice before you require your users,


In simple words: use functions, keep it open.