Read values and list of lists in Haskell

Question

Before to mark this question as duplicated, I already read this topic: Haskell read Integer and list of lists from file and the solution doesn't solve my problem.

I'm trying to read the content in a File that contains this structure:

String, String, [(Int, Int, Int)]

The file looks something like this:

Name1   22/05/2018  [(1, 5, 10), (2, 5, 5), (3, 10, 40)]    
Name2   23/05/2018  [(1, 10, 10), (2, 15, 5), (3, 50, 40),(4,20,5)]    
Name3   22/05/2018  [(4, 2, 1), (5, 2, 2), (6, 50, 3), (1,2,3)]    
Name4   23/05/2018  [(1, 3, 10), (2, 1, 5), (3, 2, 40), (6,20,20)]

In Haskell, I created this function to read the contents of the file and "convert" this content to my custom type.

rlist :: String -> [(Int, Int, Int)]
rlist = read

loadPurchases :: IO [(String, String, [(Int, Int, Int)])]
loadPurchases = do s <- readFile "tes.txt"
                   return (glpurch (map words (lines s)))

glpurch :: [[String]] -> [(String, String, [(Int, Int, Int)])]
glpurch [] = []
gplpurch ([name, dt, c]:r) = (name, dt, (rlist c)) : gplpurch r

But when I try to execute the "loadPurchases" function, I get this error: Non-exhaustive patterns in function glpurch.

Using :set -Wall, I received this help message:

<interactive>:6:1: warning: [-Wincomplete-patterns]
Pattern match(es) are non-exhaustive
In an equation for `glpurch':
    Patterns not matched:
        ([]:_:_)
        ([_]:_)
        ([_, _]:_)
        ((_:_:_:_:_):_)

My problem is how to create all these conditions.

I will be very grateful if anyone can help me create those conditions that are likely to determine the "stopping condition"

Your tips almost worked. I do this: glpurch ((name : dt : c : _) : r) = (name, dt, (rlist c)) : glpurch r, but now I get this message: [("Name1","22/05/2018",*** Exception: Prelude.read: no parse — Leonan Fraga Leonardo
– Leonan Fraga Leonardo, Commented Jun 7, 2018 at 18:44
In GHCi try words "Name4 23/05/2018 [(1, 3, 10), (2, 1, 5), (3, 2, 40), (6,20,20)]" and see what you're getting. Then type it !! 2 to see what the value of c is that is passed to rlist. Notice how that is only part of the list. — Thomas M. DuBuisson
– Thomas M. DuBuisson, Commented Jun 7, 2018 at 19:22

Thomas M. DuBuisson · Accepted Answer · 2018-06-07 19:24:07Z

6

You are only matching lists of length 3 when in fact there are many more words on each line. Just try it in GHCi:

> words "Name1   22/05/2018  [(1, 5, 10), (2, 5, 5), (3, 10, 40)]"
["Name1","22/05/2018","[(1,","5,","10),","(2,","5,","5),","(3,","10,","40)]"]

You probably want to recombine all words past the first two:

glpurch ((name : dt : rest) :r) = (name, dt, (rlist $ unwords rest)) : gplpurch r

answered Jun 7, 2018 at 19:24

Thomas M. DuBuisson

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2 Comments

Leonan Fraga Leonardo Over a year ago

Yes, that was exactly it. Another solution would be to remove the whitespace between the commas in the list?

Leonan Fraga Leonardo Over a year ago

I remove all the whitespaces in file, and solve with your and @Welperooni suggestions. Thank you very much guys.

Leonan Fraga Leonardo · Accepted Answer · 2018-06-07 19:36:45Z

0

To solve my problem, I did what @Welperooni and @Thomas M. DuBuisson suggested.

I added this code to my function:

glpurch ((name: dt: c: _): r) = (name, dt, (read c :: [(Cod, Quant, Price)

And I removed the blanks that were in the list in my file, these spaces made the division of the text not done correctly.

answered Jun 7, 2018 at 19:36

Leonan Fraga Leonardo

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