There is no Spoon
(Bending the Compiler)

Matej Kollár
mkollar+fpb0@redhat.com

February 2015

Problem

I want to keep track of books (todo, progress, done).
Basic options that come up
- Text file
  - Unstructured or Somehow structured (Markdown, ASCII Doc, …)
- Database

Text file

Pros

Simple.
Just one file.
Easy to read and edit.

Cons

Unless you have extremely developed self discipline
- easily gets messy;
- easily gets inconsistent.
Structure might not be obvious to you after some time.
Hard to change structure (what is tracked) in consistent way.
Not very smart tooling
- not easy to query (not much beyond grep).

Database

Pros

It keeps structure for you.
It helps you with consistency.
If you study schema, you will be able to remember structure even later on.
Easy to query.
(Still might be just one simple file.)

Cons

Not really that simple.
Unless you are willing to use command line, you need to create application.
Unless application is created
- not easy/simple editing;
- not easy to read.

Undecided

Schema changes are unpleasant but possible.

Compiler

Compilers are good for… compiling.

compiler :: Source -> Input -> Output -- Oversimplified...

compiler :: Source -> Maybe (Input -> Output) -- Still oversimplified

…but what if there is more to them than that?

There is no compiler!

type Source = Text
compiler :: Source -> Error (Input -> Output) -- Still oversimplified
magic = fmap (const ()) . compiler :: Text -> Error ()

data Tag = Fiction | Programming | FunctionalProgramming
data Status = Done | Reading Rational | Todo
data Language = EN | SK | CZ
data Format = TextBook | AudioBook
data Book = Book
    { name :: String
    , authors :: [String]
    , progress :: Status
    , notes :: String
    , language :: Language
    , format :: Format
    , tags :: [Tag]
    }

Pros

It is simple.
It is just one file.
Easy to read and edit.
It enforces structure on you.
It helps you with consistency.
Types will help you remember intended structure.
Easy to query.
Converting to other formats is just matter of writing appropriate catamorphism ;-).

Cons

Not suitable for huge datasets accessed by many people (not part of original problem).
It can not vacuum nor wash your clothes nor iron them.
(There probably are some more…)

Notes

Of course, you can do very similar things with other compilers as well, though there is something to Haskell that makes it well suited for this.

P.S.: For extra awesomeness do not forget to throw in some version control.

There is no Spoon
(Bending the Compiler)

Problem

Text file

Pros

Cons

Database

Pros

Cons

Undecided

Tough choice

Third option

Compiler

There is no compiler!

Pros

Cons

Notes

Questions?

There is no Spoon(Bending the Compiler)

Problem

Text file

Pros

Cons

Database

Pros

Cons

Undecided

Tough choice

Third option

Compiler

There is no compiler!

Pros

Cons

Notes

Questions?

There is no Spoon
(Bending the Compiler)