I want to write an article giving a Haskell introduction specifically to Java developers, and would like to get feedback on my implementation. Please keep in mind that I don't want to be "too clever", as too advanced Haskell concepts would only confuse the readers. On the other hand I need to show some really interesting features (e.g. that's why I decided to use Edge a a Double and a type class instance instead of a simple (String, String, Double)), else the readers would ask "And where is the advantage over Java?". So I need a good balance between clarity and "interesting stuff", showing a nice example of Haskell's expressiveness.

module Prim (
) where

import Data.List(sort, deleteBy)
import Data.Set(member, empty, insert, singleton, Set)

data Edge a = Edge a a Double deriving (Eq, Show)

instance Ord a => Ord (Edge a) where
   compare (Edge a b c) (Edge d e f) = 
      (c, min a b, max a b) `compare` (f, min d e, max d e)

prim [] = []
prim edges = loop (sort edges) [] startSet where
   startSet = singleton (startNode edges)
   startNode ((Edge node _ _):_) = node 

loop [] solution _ = solution
loop edges solution vertices = 
   let (e,x) = findNextEdge edges vertices
       vertices' = x `insert` vertices
       cyclicEdge (Edge a b _) = a `member` vertices' && 
                                 b `member` vertices' 
       edges' = filter (not.cyclicEdge) edges 
   in loop edges' (e:solution) vertices'   

findNextEdge [] vs = error ("Disjunct graph with island " ++ show vs)                               
findNextEdge (e@(Edge a b _):edges) vertices 
    | a `member` vertices = (e,b)
    | b `member` vertices = (e,a)
    | otherwise = findNextEdge edges vertices

In particular, I'm interested in these concepts:

  • Type inference
  • Laziness, immutability
  • Currying
  • Pattern matching, guards
  • ADTs and type polymorphism

1 Answer 1

  • Java developers prefer intention revealing names as opposed to mathematical single letter identifiers. So it might be better to use edge instead of e and so on.
  • Most common type parameter name in Java is T as opposed to a, so maybe using at least 't' (if not a normal name like nodeType) to prevent confusion and better explain ADTs.
  • loop might look like a syntax construct, especially because its definition follows afterwards.
  • Just a detail, but initialSet instead of startSet would be clearer to me - same holds for startNode.
  • I would replace where with let, because it would make one construct less (although idiomatic one) and it is a convention in mainstream languages to write declarations/definitions before use.
  • I would also adjust 'loop' signature, so the solution isn't between input arguments - I wold make it the last argument.
  • I wouldn't use the apostrophe ('), although I understand it is common in math and theory of programming, but among backquoutes (`) for infix operators it might be confusing.
  • I think it would help if the function composition operator (.) was surrounded with whitespace - it would better show that it is a different kind of '.' than the one in import Data.List(sort, deleteBy), which might be easier to understand for Java developers.

I am not a Haskell guru and I don't have 10+ years of Java experience, so take this as a personal opinion. It might also help quite a lot to show the same implemented in Java - the benefits of features you describe and syntax constructs would be clearer. Feel free to comment on this answer, I am open to discussion.


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