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user1898820
user1898820

Reputation: 65

Using show with a list of lists in Haskell

I'm having some trouble using show to print the rows of a matrix given by a list of lists.

I have this:

data Matrix = Mat Int [[Bit]] 
    deriving Eq

Where the argument Int is the order of the squared matrix and Bit is an Int (0 or 1). I need that my code to be able to do the following, with Matrix as an instance of Show:

Main> Mat 3 [[0,0,1],[1,0,1],[1,1,1]
[0,0,1]
[1,0,1]
[0,0,1]

So far I only have:

instance Show Matrix where
    show (Mat i (x:xs)) = (show x) ++ "\n"

But this obviously only returns the first list. Could you help me with this problem? Thanks in advance.

Upvotes: 1

Views: 571

Answers (1)

Daniel Fischer
Daniel Fischer

Reputation: 183858

The simple way is to show all rows, and put them on their own line each:

instance Show Matrix where
    show (Mat _ rows) = unlines $ map show rows

The slight drawback of that is that it also adds a newline after the last row, to avoid that, you can use

instance Show Matrix where
    show (Mat _ rows) = intercalate "\n" $ map show rows

(needs an import of Data.List for intercalate)

Upvotes: 8

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