Hi Ali,
On 23 July 2015 at 07:24, Ali Baharev
I have started learning Haskell recently, and I try to reimplement interesting algorithms that are challenging to do in other languages. I would like to ask you for your kind help with a problem that I am stuck with. ... Here is my first attempt:
data Tree a = Leaf a | Node [Tree a]
tree = Node [Leaf 'a', Node [Leaf 'b', Leaf 'c', Node []], Leaf 'd']
toString :: (Show a) => Tree a -> String toString (Leaf leaf) = " " ++ show leaf ++ " " toString (Node nodes) = " [ " ++ concatMap toString nodes ++ " ] " ... the part that I am really not happy with is the lot of code duplication in giving the tree. I would like to enter it as: tree = ['a', ['b', 'c', []], 'd'] which of course won't work. Is there a (not too obscure) way to help the compiler so that it understands that 'a' means Leaf 'a', etc?
On Wed, Jul 22, 2015 at 11:51 PM, elliot
In general you don't produce large trees (or any data structure) by hand.
I am a Haskell novice myself, so don't take my advice as expert opinion, but it sounds like you may have the wrong expectation about what Haskell offers regarding "shorter", "cleaner", "purer" code. I think Elliot was right to question your question. You seem to want a more concise way to define a constant without having to repeatedly type the data constructor names, hoping that the compiler offers some nifty syntax for saving a few characters of duplicated text. The answers by others seem to agree that "there isn't any". I think you should not focus so hard on trying to save a few characters in the definition of "tree". The way I see it, the "conciseness" aspect of functional programming is not really about saving a few characters in the small. It is more about saving thousands of lines of code in the large. You do this by using modular, composable abstractions, and, in Haskell's case, exploiting laziness. Focus instead on writing a higher-order traversal function that can be used to write "toString" and all your other tree-traversing functions as simple one-liners. To get started, you should read this influential article by J. Hughes [1][2]. The first third of the article demonstrates these ideas with lists and trees. The article is well-suited to imperative programmers learning their first functional language. You don't need a doctorate to understand it. This should give you some hints on how to exploit Haskell better in your own list and tree implementations. (Hughes' tree data type is slightly different from yours; his has labels on all the nodes, not just the leaves, but the lesson is still perfectly relevant.) [1] Hughes, J. "Why Functional Programming Matters", The Computer Journal 32 (2), pp. 98-107, 1989. [2] http://www.cse.chalmers.se/~rjmh/Papers/whyfp.html -- Thomas Koster