Scala sample code for Bartosz Milewski's Category Theory for Programmers

WIP

Contributing

Any contribution (PR, review, issues) will be more than welcome! If you are interested, please indicate that you are working on a certain section in the respective issue. You can find the issues here..

To compile

sbt tut

Style guide

Use ```tut:silent``` for code that is compilable, use ```scala``` otherwise.

Use a line of ............. to separate between code examples.

There are multiple ways to translate haskell code into scala. For scala syntax, generally speaking we follow the common styles in FP scala codebase (e.g. typelevel/cats)

functions vs. methods

If it's a "standalone" function, use the function syntax, i.e.

val f: A => B = ???

If it is inside a trait or class, use the method, i.e.

def f(a: A) : B = ???

Currying

Follow the common strategies in FP codebases, roughly

• if there is one function parameter, create a dedicated parameter list for it and make it the last parameter list.
• if there are multiple function parameters, group them together, and put them in a dedicated last parameter list.
• of course, implicit parameters have to go in a separate parameter list
• everything else should go to a single parameter list.
• if the haskell code partially applied the function, partially apply with underscore in scala code.

Infix syntax for type class methods

Create a XXXOps implicit extension class to support such infix syntax. E.g. To support (m1: A => Writer[B]) >=> (m2: B => Writer[C]) add

  implicit class Kleisli[A, B](m1: A => Writer[B]) {
   def >=>[C](m2: B => Writer[C]): A => Writer[C] = ...

Indentation and line break

Use 2 space indentation and prefer to aggressively break lines

def >=>[C](m2: B => Writer[C]): A => Writer[C] 
   = { x => 
     ...
   }

If the signature is too long, we break it to multiple lines as well. Control each line to be within 45 characters.

Symbol names

For the prime symbol, use U+16CC RUNIC LETTER SHORT-TWIG-SOL S ᛌ, it works with scalac while the unicode prime symbol doesn't

License

This work is licensed under a Creative Commons Attribution 4.0 International License.