Add default introductory code example

[kytrinyx]

Instead of having logic for a fallback in the backend, we're choosing a default for the snippet file.

For tracks that have core exercises, we pick the first core exercise. For tracks without these, we pick the first exercise listed in the config.

Note that we're aiming for 10 lines and a max-width of 40 columns. This solution has a max-width of 61, so we may want to adjust it.

See https://github.com/exercism/meta/issues/89

[kytrinyx]

I think we should just remove the conditional. I don't actually know Idris so I hesitate to mess with this (I know I can figure it out, but I'd totally rather accept someone else's PR and close this one). If you do submit something, would you mind @mentioning me or dropping a line in here so I get the notification?

[yurrriq]

How do you mean, conditional? IIUC we could do something like one of the following:

• 44x7 wide

  module HelloWorld
  
  export
  greet : Maybe String -> String
  greet (Just name) = "Hello, " ++ name ++ "!"
  greet Nothing     = greet "World"

• 37x6 wide

  module HelloWorld
  
  export
  greet : String -> String
  greet name = "Hello, " ++ name ++ "!"

• 35x10

  module Leap
  
  export
  isLeap : Int -> Bool
  isLeap year =
    (divBy 4 && not (divBy 100)) ||
    divBy 400
  where
    divBy : Int -> Bool
    divBy n = (year `mod` n) == 0

• 29x5, http://docs.idris-lang.org/en/latest/tutorial/starting.html

  module Main
  
  main : IO ()
  main = putStrLn "Hello world"

[kytrinyx]

How do you mean, conditional

Sorry, I mean the bit where it's either "world" or the name that gets passed in:

greet subject = "Hello, " ++ fromMaybe "World" subject ++ "!"

Do you have a preference? I'm leaning towards the 37x6 wide one, since it fits, but if you think it doesn't do a good job showing off the language, the Leap one would also be great.

[yurrriq]

I'm cool with whatever, but none of these examples really show off the language. I'm thinking an isEven along the lines of this discussion could be good. I'll write something up later or tomorrow.

[kytrinyx]

none of these examples really show off the language.

Yeah, that's what I was worried about.

I'll write something up later or tomorrow.

Sweet, thanks!

[yurrriq]

After golfing for a while, I've decide to borrow (and slightly modify) the following from @puffnfresh's blog for a nice, 37x10 example that gives a taste of the powerful type system:

module EvenOdd

data Even : Nat -> Type where
  EZ : Even Z
  ES : Even n -> Even (S (S n))

total
ee : Even n -> Even m -> Even (n + m)
ee EZ m     = m
ee (ES n) m = ES (ee n m)

If at all possible, we should link the blog (assuming that's cool), since this probably doesn't mean much on its own to newcomers..

[kytrinyx]

Could we have a shortened URL as a comment in the code example?

[kytrinyx]

(Also: this is a great code example!)

[puffnfresh]

I don't need attribution, just ship it :heart: :ship:

[kytrinyx]

@puffnfresh that's awesome thank you. 🚢 'd it!

[yurrriq]

My PR was for this branch, so I'm not sure it got merged..

[kytrinyx]

D'oh. That's what I get for being too quick.

[kytrinyx]

Let's try this again.

[yurrriq]

LGTM