Question

Trying to implement a function that will return a list of ints the represent an ordering of each list of doubles, e.g.:

orderings [ [1.0, 2.0, 3.0], [3.0, 2.0, 1.0] ]
> [ [0, 1, 2], [2, 1, 0] ]

However, having trouble with my pattern matching for some reason:

import Data.List

-- Return a list of orderings for each list of doubles
orderings:: [[Double]] -> [Int]
orderings [] = []
orderings x:xs = (ordering x):(orderings xs)

ordering:: [Double] -> [Int]
ordering xs = [i | (i, _) <- sorted] where
    sorted = sortBy (\(i1, e1) (i2,e2) -> compare e1 e2) $ zip [0..] xs

Error is:

Parse error in pattern: orderings

Can’t see the error for the life of me!

Answer 1

Two more problems (in addition to the missing parentheses around x:xs):

1. the type of orderings is wrong; I suspect it should be [[Double]] -> [[Int]]

2. x is not in scope in ordering; I suspect it should be xs

Here’s the corrected code:

import Data.List

-- Return a list of orderings for each list of doubles
orderings:: [[Double]] -> [[Int]]  -- changed type
orderings [] = []
orderings (x:xs) = (ordering x):(orderings xs)

ordering:: [Double] -> [Int]
ordering xs = [i | (i, _) <- sorted] where
    sorted = sortBy (\(i1, e1) (i2,e2) -> compare e1 e2) $ zip [0..] xs -- xs not x!