How to get value from Either?

Question

I have this code in ghci,try to decode base64 code:

let a=pack "MTExMTEx"
let b=decode a
:t b
b :: Either String ByteString

so how to get the decode bytestring from either?is there some func like Maybe's fromJust?I can't find it,thanks.

Answer 1

Use case:

case decode a of
  Left err -> {- what to do if decode gave an error -}
  Right msg -> {- what to do if decode succeeded -}

The either function that Alexandre suggests is essentially the same as this, the two branches are just taken as functions instead; i.e. it's equivalent to write:

either 
  (\err -> {- what to do if decode gave an error -}) 
  (\msg -> {- what to do if decode succeeded -}) 
  (decode a)

Answer 2

You can use the either function from Data.Either.

Its signature is:

either :: (a -> c) -> (b -> c) -> Either a b -> c

It means it takes two functions as inputs: the first one to be applied in case it's a Left, and the second one to be applied if it's a Right. The third parameter is your Either data type. Notice that the type of the return value of both functions must be the same.

Answer 3

You're looking for Data.Either's fromRight which has a type signature

fromRight :: b -> Either a b -> b

the first value is a default value (what you'll get back if you have a Left instead of a Right.

fromRight 'a' (Left "b")
-- gives 'a'

fromRight 'a' (Right 'c')
-- gives 'c'