CODEWITHSUNDEEP
haskelleither
thor
thor

Reputation: 22460

How to extract a Either's Right and keep information about its Left in case of error in Haskell?

I am trying to extract the value in the Right constructor of a Either value, while giving an error if the Either in question is actually a Left (i.e. an error). The answers in Either Right Left how to read value gives me something like:

fromRight e = either (const $ error "Either Left encountered while expecting Right") id e

This works but discards useful information in the error message of the Left ctor of Either. How can I post an error message about the Left instead?

-- EDIT --

Thanks for the input. I wanted this as a more informational version of fromJust.

Also, I'd like to avoid writing a case statement every time, and want to avoid Monads whenever it's not too complicated (to keep the function "eval" style). For my use case, it's computation-oriented, and errors occur only when something like invalid input was supplied (when there is no remedy).

I ended up using:

fromRight e = either (error.show) id e

Upvotes: 0

Views: 925

Answers (2)

dfeuer
dfeuer

Reputation: 48581

Instead of using const ... in the first argument of either, use something else.

either oops .... where
  oops x = error $ "Oops! Got an " ++ show x

Or whatever.

Note, however, that error should only be used for internal errors. User errors, connectivity errors, etc., should be allowed to bubble out to IO and then reported with throwIO or handled gracefully.

Upvotes: 4

Frerich Raabe
Frerich Raabe

Reputation: 94289

Instead of

const $ error "Left encountered"

you can use a lambda to get the value and use it, e.g.

\v -> error $ "Left encountered: " ++ v

Upvotes: 3

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