How to seperate Array in Haskell-CodePudding

i just learn about haskell but i've problem about Type Data, ihave type data like below

[Test {name = "name1", Address = "Address1", Score = 80},Test {name = "name2", Address = "Address2", Score = 60}]

my question is how to seperate to =

Name  Address  Score Pass
-------------------------
name1 Address1 80    Y
name2 Address2 60    N

i've try

printArray :: [Test] -> IO()
printArray [Test x y z]  = do
                                putStr x
                                putStr (" "    y)
                                putStr (" "    show z)
                                if z >= 70 then putStrLn " Y"
                                else putStrLn " N"

but when i executed i got error : Non-exhaustive patterns in function printArray

Thanks in advance

CodePudding user response：

There aren't any arrays here. [] is for lists.

The reason your code with printArray [Test x y z] = ... doesn't do the trick is that it only matches the case of a list with exactly one element in it. If you want to do this for all elements you can use either

Recursion.

printTests (Test x y z : tests) = do
              putStr x
              putStr (" "    y)
              putStr (" "    show z)
              putStrLn $ if z >= 70 then " Y"
                                    else " N"
              printTests tests
printTests [] = return ()

A standard looping combinator

printTests = mapM_ $ \(Test x y z) -> do
               ...

...or, perhaps easier to understand

import Control.Monad (forM_)

printTests tests = forM_ tests $ \(Test x y z) -> do
               ...

(Recommended) Separate your concerns: instead of doing nasty IO in a loop setting, start with simple
```
showTest :: Test -> String
```
And then use putStrLn (unlines $ map showTest tests) where printing actually is needed.

CodePudding user response：

This is wrong. You pattern-match the singleton list containing just one element [Test x y z].

printArray :: [Test] -> IO ()
printArray []  = pure ()
printArray (Test x y z : xs)  = do
  putStr x
  putStr (" "    y)
  putStr (" "    show z)
  if z >= 60
    then putStrLn " Y"
    else putStrLn " N"
  printArray xs

CodePudding user response：

Split it:

printTest :: Test -> IO()
printTest (Test x y z) = do
  putStr x
  putStr (" "    y)
  putStr (" "    show z)
  if z >= 60
    then putStrLn " Y"
    else putStrLn " N"

printArray :: [Test] -> IO()
printArray [] = pure ()
printArray (t:ts) = do
    printTest t
    printArray ts