Either Right Left how to read value

Question

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]]

score 3 · Accepted Answer · answered Nov 07 '13 at 15:04

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

score 1 · Answer 2 · answered Nov 07 '13 at 15:01

1

Change it to

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

answered Nov 07 '13 at 15:01

Rob Stewart

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Thanks that worked well. Just small typo let (Right result) = parseCSV contents – Tom Nov 07 '13 at 15:05
1

This will give a runtime pattern-matching error if the result *isn't* `Right`, however. Be warned. – J. Abrahamson Nov 07 '13 at 15:16

score 1 · Answer 3 · answered Nov 07 '13 at 15:02

1

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.

answered Nov 07 '13 at 15:02

Ingo

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Either Right Left how to read value

3 Answers3

Linked