Non Exhaustive Patterns in function

Question

I'm writing a program in Haskell that can pretty print a table and do basic queries on it. The following function is a snippet of the code which prints a table:

printTable :: Table -> [String]
printTable table@(header:rows) = [addLine] ++ addHeader ++ [addLine] ++ addRows rows ++ [addLine]
    where widthList            = columnWidths table
      makeTupleList []         = []
      makeTupleList (x:xs)     = zip widthList x : makeTupleList (xs)
      addRows line             = map printRow (makeTupleList line)
      addLine                  = printLine widthList
      addHeader                = addRows [(map.map) toUpper header]

Note: Table == [[String]]

After calling this function with the 'unlines' function, the table is printed.

If I test this function, giving it a [[String]] argument, it works correctly. However, if I test this function in my 'main' code, I get the error:

Non-exhaustive patterns in function printTable

The only difference is that in my main code, the user of the program can give a text file as an input:

main :: IO()
main = interact (lines >>> exercise >>> unlines)

exercise :: [String] -> [String]
exercise = parseTable >>> select "gender" "male" 
                  >>> project ["last", "first", "salary"] >>> printTable

Any help to solve this problem is more than welcome!

Answer 1

When you pattern match on (x:xs), it will only match if there is at least one item in the list.

You need to handle the case of an empty Table parameter.

printTable [] = ...