[Haskell-cafe] Haskell - convert postfix expression to binary tree

[Brandon Allbery]

It would help if you included the full error, which shows which pattern it
thinks is missing. Just looking at the code, one pattern I don't see is
(add Empty [] _). And I think I see two other missing patterns which are
more complex.

On Mon, Aug 21, 2017 at 12:10 PM, Martin Studna <martin.studna2 at gmail.com>
wrote:

> Hello,
>
> I am trying to convert postfix expression to binary tree. My function
> takes as argument a list of tokens (strings).
>
> Everytime I give the function any input, debugger writes a message:
> Non-exhaustive patterns in function "add".
>
> My idea was: read a token after token and determine, if it is an operator
> or an operand. If it is operand, don't save any node to the tree and store
> the number to the stack. Otherwise I create a node with an operator, pop
> symbols from stack, set them as children of new node and push the operator
> to stack.
>
> If the list of strings is empty, functions print the binary tree.
>
> Would someone explain to me, why the function gives non-exhaustive
> patterns error and how can I fix the function?
>
> data Tree = Leaf String | Empty | Node Tree String Tree deriving (Show)
>
> add :: Tree -> [String] -> [Tree] -> Tree
> add (Node l v p) [] stack = (Node l v p)
> add Empty (x:xs) []
>         | x `elem` ["*","-","+"] = add (Leaf x) xs [Leaf x]
>         | otherwise = add Empty xs [Leaf x]
> add Empty (x:xs) (a:b:bs)
>         | x `elem` ["*","-","+"] = add (Node b x a) xs (Leaf x:a:b:bs)
>         | otherwise = add Empty xs (Leaf x:a:b:bs)
> add (Leaf x) token (a:b:bs)
>         | x `elem` ["*","-","+"] = add (Node b x a) token (Leaf x:bs)
>         | otherwise = Leaf x
> add (Node l v p) (x:xs) (a:b:bs)
>         | x `elem` ["*","-","+"] = add (Node b x a) xs (Leaf x:bs)
>         | otherwise = add (Node l v p) xs (Leaf x:a:b:bs)
>
> parse :: String -> Tree
> parse input = add Empty (words (toPostfix input)) []
>
>
> _______________________________________________
> Haskell-Cafe mailing list
> To (un)subscribe, modify options or view archives go to:
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> Only members subscribed via the mailman list are allowed to post.
>



-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20170821/a08d086b/attachment-0001.html>