On Tue, Apr 14, 2015 at 11:47 AM, Shishir Srivastava <shishir.srivastava@gmail.com> wrote:

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.

Because that's the way Either was made an instance of fmap. It's defined so that fmap _ (Left x) = Left x, but fmap f (Right y) = Right (f y).

The docs say:

The 'Either' type is sometimes used to represent a value which is

either correct or an error; by convention, the 'Left' constructor is

used to hold an error value and the 'Right' constructor is used to

hold a correct value (mnemonic: \"right\" also means \"correct\").