CODEWITHSUNDEEP
haskell
Tom
Tom

Reputation: 93

Either Right Left how to read value

I am trying to read CSV file which I succeded with some examples. Here is what I've got

*Main System.IO> let result=parseCSV contents
*Main System.IO> result
Right [["Name","Value","Amount"],["Rob","1","10"],["Bob","1.42","15.3"],["Tom","452.2","23.1"]]

But If I try to read value from this array I get an error

*Main System.IO> head result

<interactive>:21:6:
    Couldn't match expected type `[a0]'
            with actual type `Either ParseError [[String]]'
In the first argument of `head', namely `result'
In the expression: head result
In an equation for `it': it = head result

So how can I get Rid of the Right and actually use the list ?

*Main System.IO> :t result
result :: Either ParseError [[String]]

Upvotes: 4

Views: 2754

Answers (3)

Cirdec
Cirdec

Reputation: 24166

To get rid of the Right, you need to decide what to do when you get an error instead. If you just want to get an empty result you could use:

justTheResult :: [[[Char]]]
justTheResult = either (const []) id result

If you want an error to be raised, you could use:

justTheResult :: [[[Char]]]
justTheResult = either (const $ error "Couldn't parse CSV") id result

If you want to keep the error for later, but do something with the result, you could instead use Either's Functor (Either e) instance:

errorOrNumberOfRows :: Either ParseError Int
errorOrNumberOfRows = fmap length result

Now you'll be in the same place accessing that Int length

Upvotes: 3

Ingo
Ingo

Reputation: 36349

If you really want to ignore the error, the following shoud do the job:

either (const []) id (parseCSV contents)

It will give you an empty list in case of parse errors.

Upvotes: 1

Rob Stewart
Rob Stewart

Reputation: 1842

Change it to

*Main System.IO> let (Right result) = parseCSV contents
*Main System.IO> head result

Upvotes: 1

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