Re: [Haskell-beginners] Left vs Right

If you really do want to apply a function over data in a Left, there's always `Control.Arrow.left`, which is the complement of `fmap` in this case: λ left (replicate 3) (Left "blah") Left ["blah","blah","blah"] λ left (replicate 3) (Right "blah") Right "blah" Or you can use Either's Bifunctor instance: λ import qualified Data.Bifunctor as Bi λ Bi.first reverse (Left "foo") Left "oof" λ Bi.first reverse (Right "bar") Right "bar" λ Bi.second (intersperse '!') (Left "papaya") Left "papaya" λ Bi.second (intersperse '!') (Right "banana") Right "b!a!n!a!n!a" λ Bi.bimap sort reverse (Left "hello") Left "ehllo" λ Bi.bimap sort reverse (Right "goodbye") Right "eybdoog" On Tue, Apr 14, 2015 at 9:58 AM, Chas Leichner wrote:

This is meant to model errors. You want your computation to continue if everything is going all-Right, but if there is an error, you want it to stop there are report the error. This error is represented by Left constructor.

On Tue, Apr 14, 2015 at 9:56 AM, Bob Ippolito wrote:

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The Functor instance is defined for `Either a`, so Left is fixed. fmap only maps over Right. You'll find the same behavior for `(,) a`, e.g. `fmap (*2) (1,2) == (1,4)`.

On Tue, Apr 14, 2015 at 9:47 AM, Shishir Srivastava < shishir.srivastava@gmail.com> wrote:

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Hi,

Can someone please explain the difference in outputs of the following two expressions -

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ghci> fmap (replicate 3) (Right "blah") Right ["blah","blah","blah"]

ghci> fmap (replicate 3) (Left "foo") Left "foo"

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Why does 'Right' return a list of Strings whereas 'Left' returns just a String.

Thanks, Shishir

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