error while creating a custom max function haskell

Question

I was trying to create max function to compare a list of n dimensional coordinates based on the values of 1st coordinate

max :: [(Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int)] -> (Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int)
max []                               = (0,0,0,0,0,0,0,0,0,0,0,0,0,0)
max [(ar,a,b,c,d,e,f,g,h,i,j,k,l,m)] = (ar,a,b,c,d,e,f,g,h,i,j,k,l,m)
max (ar,a,b,c,d,e,f,g,h,i,j,k,l,m):(ar1,a1,b1,c1,d1,e1,f1,g1,h1,i1,j1,k1,l1,m1):tail
    | ar > ar1                       = max (ar,a,b,c,d,e,f,g,h,i,j,k,l,m):tail
    | otherwise                      = max (ar1,a1,b1,c1,d1,e1,f1,g1,h1,i1,j1,k1,l1,m1):tail

But I am getting error in 4th line:

1.hs:81:1: error: Parse error in pattern: max
    |
81 | max (ar,a,b,c,d,e,f,g,h,i,j,k,l,m): 
(ar1,a1,b1,c1,d1,e1,f1,g1,h1,i1,j1,k1,l1,m1):tail    | 
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

shree.pat18 · Accepted Answer · 2018-11-30 08:28:56Z

4

A couple of things to fix it:

In your case for multi-item list, surround the argument with brackets i.e.

max ((ar,a,b,c,d,e,f,g,h,i,j,k,l,m):ar1,a1,b1,c1,d1,e1,f1,g1,h1,i1,j1,k1,l1,m1):tail)

When you make the recursive call, qualify your function name. Else, you will end up with ambiguity between Prelude.max and your own function. I'd recommend changing the function name as the easy solution.

Taken together, you should see something like this:

max2 :: [(Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int)] -> (Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int)
max2 []                               = (0,0,0,0,0,0,0,0,0,0,0,0,0,0)
max2 [(ar,a,b,c,d,e,f,g,h,i,j,k,l,m)] = (ar,a,b,c,d,e,f,g,h,i,j,k,l,m)
max2 ((ar,a,b,c,d,e,f,g,h,i,j,k,l,m):(ar1,a1,b1,c1,d1,e1,f1,g1,h1,i1,j1,k1,l1,m1):tail)
    | ar > ar1                       = max2 $ (ar,a,b,c,d,e,f,g,h,i,j,k,l,m):tail
    | otherwise                      = max2 $ (ar1,a1,b1,c1,d1,e1,f1,g1,h1,i1,j1,k1,l1,m1):tail

answered Nov 30, 2018 at 8:28

shree.pat18

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1 Comment

talex Over a year ago

Also you can give (ar,a,b,c,d,e,f,g,h,i,j,k,l,m) alias like this someIdentifier@(ar,a,b,c,d,e,f,g,h,i,j,k,l,m) and use it instead of typing all the names again.

talex · Accepted Answer · 2018-11-30 09:10:04Z

2

You can use this code

import Data.List(maximumBy)
import Data.Ord(comparing)

max2 :: [(Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int)] -> (Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int)
max2 [] = (0,0,0,0,0,0,0,0,0,0,0,0,0,0) 
max2 x  = maximumBy cmp x   
    where 
       cmp = comparing first
       first (v,_,_,_,_,_,_,_,_,_,_,_,_,_) = v

answered Nov 30, 2018 at 9:10

talex

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2 Comments

chepner Over a year ago

That type screams out for an alias as well :) type FourteenInts = (Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int).

Tamil Selvan Over a year ago

well this is used only once and there are 12ints , 10ints, 8ints and 6ints so thought of writing it directly :P

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