a field guide to functional programming

Where functional programming puts down roots.

Pull up a chair at the potting table. Here, Haskell concepts grow like heritage plants: functors follow the sun, monads climb their trellises, and lazy evaluation unfurls only when the morning is ready for it.

λ as vine, proof as garden path.

preparing the soil

Haskell basics, made earthy.

Every Haskell garden begins with dependable ground. Values do not wriggle into new shapes behind your back; once planted, they stay themselves. Functions are pure, too: give them the same compost of inputs and they return the same bloom every time.

The compiler is not a scolding crow. It is your patient almanac, catching frost warnings before you lose the crop. When a signature says a function takes a String, it means a String — not a surprise turnip.

greet :: String -> String
        That double colon is a promise label on the seed packet. You can read it before the program runs and know exactly what sort of plant you are tending.



    
      
        
        A functor is a flower pot: change what is inside without shattering the pot.
        Variety: FunctorFollows the sun of fmap.
      
      
        planting seeds
        Types, functors, and the catalogue drawer.
        Type classes are the oak drawers of your seed catalogue. They do not say what a plant is made of; they say what it knows how to do. A Functor knows how to map sunlight through a container without disturbing the shape of the bed.
        fmap :: Functor f => (a -> b) -> f a -> f b
        You provide the transformation; Haskell keeps the structure. A list remains a row, a Maybe remains a little covered pot, and nobody steps on the seedlings.
      

    


    
      
        
        Composition is a trellis: one sturdy function supports the next.
        Variety: CompositionPrune nested calls into clean vines.
      
      
        tending the garden
        Functions that climb together.
        Once the beds are growing, you begin to train the vines. Function composition lets small, honest functions support each other, so your program reads like a path from gate to greenhouse.
        normalize :: String -> [String]
        It is not magic; it is tidy gardening. Each tool does one job, and the composition operator . ties those tools with twine.
      

    


    
      
        the greenhouse
        Hover through the type garden.
        Effects in Haskell are kept under glass: visible, cared for, and not allowed to run through the beds uninvited. Hover each specimen to reveal its type signature.
      
      
        
        
        
        
      
      Let your cursor wander over a plant; its habit will appear here like a label written in pencil.
    

    
      
        
        Lazy evaluation is the fern that unfurls only as far as you ask it to.
        Variety: LazyWater on demand; no wasted dew.
      
      
        harvest
        A hand-drawn map of the type garden.
        By harvest time, the beds connect. Base types are root vegetables, functors flower above them, applicatives build frames, and monads guide effects along the path without trampling purity.
        
        main :: IO ()
  putStrLn "What shall we plant today?"
  seed <- getLine
  putStrLn ("A fine bed for " ++ seed ++ ".")
        The do block is a tidy greenhouse bench: effects happen in sequence, while pure expressions remain neatly labeled and easy to lift.