The prevalent wisdom seems to be that FSharp is only for Math, Science and Engineering. F# for Boring Line of Business Apps aims to show why it's capable of far more than that.

1. F# for BLOBA
It isn’t just for Science and Financial
Domains
Boring Line Of Business Applications

2. Brandon D’Imperio
imaginarydevelopment.blogspot.com
@maslowjax

3. A Team of 3 Devs Beat a Team of 8
A team that peaked at 3 developers (only one
of which knew F#) finished in 1 year what a
team that peaked at 8 developers couldn’t do in
5.

4. But That’s Just One Company Rave
● @TachyusCorp - From zero to shipping
product in 12 weeks ( $28 million within 26
months)
● Princeton University - F# ... made it trivial ...
( Graduate courses on Parallelism)

5. Define Win
Implementation C# F#
Braces 56,929 643
Blanks 29,080 3,630
Null Checks 3,011 15
Comments 53,270 487
Useful Code 163,276 16,667
App Code 305,566 21,442
Test Code 42,864 9,359
Total Code 348,430 30,801

6. LoC is a Terrible Metric
In most ways yes.
C#, F#
● Test Code Ratio 14%, 44%
● Useful Code Ratio 53%, 78%
o Almost half the c# code was noise
● The C# was not finished and often failed to
reach near-real-time

7. What Does This Mean to Me?
● Could you find bugs more easily in 1/5th of
the code? 1/10th?
● OOP SOLID and Design patterns become a
thing of the past
o Design patterns are language bug reports -
(paraphrased) - Peter Norvig
o DI is not even relevant

8. A Code Break - What’s Wrong? (with both)
C#
public class Contact {
public string FirstName {get;set;}
public string MiddleInitial {get;set;}
public string LastName {get;set;}
public string EmailAddress {get;set;}
// true if ownership of email address
is confirmed
public bool IsEmailVerified {get;set;}
}
F#
type Contact = {
FirstName: string
MiddleInitial: string
LastName: string
EmailAddress: string
// true if ownership of email address
is confirmed
IsEmailVerified: bool
}

9. What’s Wrong?
Which are optional?
What are the constraints? (are all strings allowed to be
unlimited length?)
Which fields have a relationship?
What is the domain logic?

10. Desired Features of BLOBA Development
● Express requirements clearly

11. Desired Features of BLOBA Development
● Express requirements clearly - Is the flow of
the code clearer with less noise?

12. Desired Features of BLOBA Development
● Express requirements clearly
● Rapid development cycle
o Built-in REPL
o Far less boilerplate code
o Production-ready code capability for non-f# devs in
2-4 weeks

13. Desired Features of BLOBA Development
● Express requirements clearly
● Rapid development cycle
● High quality deliverables
o Why don’t we sub-type ints? (Primitive Obsession)
o Is that int the Id of a user or the Id of something
else?

14. Desired Features of BLOBA Development
● Express requirements clearly
● Rapid development cycle
● High quality deliverables
● Fun (Ruby anyone?)

15. Features not to be missed
● No null
● Extended strong typing
● Enums (DU), Records (no more mapping
code), Units of Measure, Strongly typed
strings!
o Type Providers
o Design-time checking of string formats

16. Units of Measure
[<Measure>]
type MemberId
[<Measure>]
type m // meters
[<Measure>]
type mile
let mPerMile: float<m/mile> = 1609.34<m/mile>
let milesToMeters (mi:float<mile>) = mi * mPerMile

17. Features We’d Like to Forget
● Inheritance
● Mutability
● Accidental Recursion
● Cyclic Dependencies
o In C#
 Very do-able via Web References
 Very do-able within an assembly

18. Features Worth Mentioning
● Pattern matching
● Composition over inheritance
● typedefof<> and typeof<>

19. Methods
C#
public void Main(string[] args){ /* … */ }
F#
● let Main (args: string[]) = ...
● let Main (args: string array) = …
● let Main args = ...

20. Method calling options
let HelloWorld a b c = …
let HelloWorld (a,b,c) = …
let HelloWorld (a,b) c = … // different
// more concrete
let ProcessName first m last = …

21. More stats
● Compiler comparison
o LoC
 Roslyn C# - almost 500k lines of code
 F# - 150k lines of code
 over half the C# lines are non-useful
o Cycles
 899 mutually dependent classes vs ZERO

22. Warts
● (args:string[]) [] means array here every other context in F# [ ] would be a
list.
● required fun! fun e -> e instead of C#’s e => e
● printfn with [<Measure>] (fixed in next version most likely)
● this. everywhere inside a type
● no protected members
● Property syntaxes ( immutable, mutable) (auto option)
o member this.MyProp = foo //immutable
o member this.MyProp with get() = … // mutable
o member val MyProp = “” // auto immutable
o member val MyProp = “” with get,set // auto mutable

23. Links
● My blog
o http://imaginarydevelopment.blogspot.com/
o Why am I so enthusiastic about F#? -
http://imaginarydevelopment.blogspot.com/2014/06/why-am-i-so-enthusiastic-about-f.html
● F# cheat sheet - http://dungpa.github.io/fsharp-cheatsheet/
● Domain Driven Design using the F# type system -
https://www.slideshare.net/slideshow/embed_code/28992749 - Scott Wlaschin
● Does the language you use make a difference (revisited) -
http://simontcousins.azurewebsites.net/does-the-language-you-use-make-a-difference-revisited/
● What about inheritance? - http://codewonderings.blogspot.com/2012/09/what-about-inheritance.
html
● SOLID: the next step is Functional http://blog.ploeh.dk/2014/03/10/solid-the-next-step-is-functional/
- Mark Seemann