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F# and Design principles - SOLID

·4 mins

SOLID and its relevance to F# #

There has been an increasing amount of exposure for F# and functional programming lately.  If you come from an object-orientated background a change in mindset is required when working with functional programming, there is a lot of misinformation on functional languages and their relationship with object-orientated design. In this post we run quickly through SOLID to see if these object-orientated principles apply to F#, and if so, how.

This post assumes you are familiar with SOLID principles, if not here is a [link](

Lets take a quick overview of what SOLID stands for:

Single responsibility principle #

This is the notion that an object should have only a single responsibility.

An object-orientated program consists of layers of abstract classes with less abstract classes layered on top of ones that are more abstract.  Functional programming is similar, although abstractions are used throughout the design and are composed into a final solution.  Programming in F# naturally forms small succinct functions which should have a single purpose, so the single responsibility rule holds strong here.

Open closed principle #

The notion that “software entities should be open for extension, but closed for modification”

This comes down to building abstractions via inheritance, and behavior changes through polymorphism.  Early on a decision must be made on which parts will change, and which will be fixed.   The open closed principal is geared towards languages where inheritance is a core concept, inheritance and polymorphism are not strongly used in F#, so this principle is very weak here.  Composition and Type augmentation are the core methods for extension in F#.

Liskov substitution principle #

Liskov substitution states “objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program”.

This principle is all about inheritance - derived types preserving the specification of base types.  Functional languages like F# do not always use inheritance, it is used quite rarely and only in certain situations.   The idioms in the language like referential transparency lean strongly towards enforcing this principle.  Functional languages also have a heritage in mathematics and algebraic reasoning so referential transparency is key in this respect.

Interface segregation principle #

The notion that “many client specific interfaces are better than one general purpose interface.”

If you are violating single responsibility then your interface will probably be bloated too with unnecessary properties and methods.  The same rule applies to F#, keep your interfaces modular and keep indifferent concepts separated.

Dependency inversion principle #

The notion that one should “Depend upon Abstractions. Do not depend upon concretions.”

  1. High-level modules should not depend on low-level modules. Both should depend on abstractions.
  2. Abstractions should not depend upon details. Details should depend upon abstractions. Dependency inversion is often solved with concepts such as dependency injection; common techniques for this involves things like constructor injection and interface injection.  Inversion of control is often solved with a factory pattern or a service locator.  From a functional point of view, these containers and injection concepts can be solved with a simple higher order function, or hole-in-the-middle type pattern which are built right into the language.

Summary #

So we can assume that in the functional paradigm:

  • Types should have only a single responsibility.
  • The open closed principle doesn’t apply in the majority of cases unless we are using an object-oriented approach.  With functional programming we favour functional composition and type augmentation.
  • The same can be said of the Liskov substitute principle, pure functions uphold this tenant via immutability and referential transparency.
  • Interface segregation also applies and idiomatic F#  produces small concise functions with modular interfaces with a separation of concerns.
  • Dependency inversion is not a relevant issue for F# as the language itself supports higher-order-functions to encapsulate this concept. So only the ‘S’ and the ‘I’ parts are relevant in functional programming.  The other tenants are not fully representative as they stand. We could probably explore the tenants of good functional design and form a quirky acronym in the process, but Ill leave that for another time.

Until next time…