Mapping over Either's Left

Question

Somewhere in my app I receive an Either ParserError MyParseResult from Parsec. Downstream this result gets some other parsing done over using other libs. During that second phase of parsing there also may occur some kind of error which I would like to pass as a Left String, but for that I need to convert the result from Parsec to String too. To achieve that I need a function which will allow me to map over a Left with a show function.

The mapping function I'm thinking of looks something like this:

mapLeft :: (a -> b) -> Either a c -> Either b c
mapLeft f (Left x) = Left $ f x
mapLeft _ x = x

But I was quite surprised not to find anything matching on hackage db. So now I'm having doubts whether I'm using a correct approach to my problem.

Why isn't there such a function in standard lib? What is wrong with my approach?

You can't use `mapLeft _ x = x`, that must be `mapLeft _ (Right x) = Right x`, the argument and result have different types. — Daniel Fischer, Nov 22 '12 at 00:01

score 34 · Accepted Answer · answered Nov 22 '12 at 00:02

34

We have such a function in the standard libraries,

Control.Arrow.left :: a b c -> a (Either b d) (Either c d)

is the generalisation to arbitrary Arrows. Substitute (->) for a and apply it infix, to get the specialisation

left :: (b -> c) -> Either b d -> Either c d

There is nothing wrong with your approach in principle, it's a sensible way to handle the situation.

answered Nov 22 '12 at 00:02

Daniel Fischer

174,737
16
293
422

1

Oh. I'm not surprised I didn't even suspect it of being what I needed. My mind wasn't ready to see the arrow as a type ) Thanks for opening my eyes! – Nikita Volkov Nov 22 '12 at 00:13
4

`lens` also provides this function, by the name `over traverseLeft`. – shachaf Nov 22 '12 at 01:31
5

It is `over _left` in recent versions -- in case someone comes across this later. – Edward KMETT Nov 29 '12 at 07:22

score 20 · Answer 2 · edited Jun 28 '19 at 18:08

20

Another option is to use Bifunctor instance of Either. Then you have

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

(Also Bifunctor can be used to traverse over the first part of (a,b).)

edited Jun 28 '19 at 18:08

Chris Stryczynski

19,899
28
104
198

answered Nov 22 '12 at 08:11

Petr

60,177
8
136
295

Seeing `Either` as a `Bifunctor` in a context that, as stated, what I needed was to "map", seems very reasonable. Thank you! I wonder why Hoogle doesn't include any of these functions in its results. – Nikita Volkov Nov 22 '12 at 15:37
3

@NikitaVolkov I was curious about that too, and I found out that [Hoogle searches: array, arrows, base, bytestring, Cabal, cgi, containers, directory, filepath, haskell-src, HUnit, mtl, old-locale, old-time, packedstring, parallel, parsec, pretty, process, QuickCheck, random, stm, template-haskell, time, xhtml](http://www.haskell.org/haskellwiki/Hoogle#Scope_of_Web_Searches). Probably one would have to create a full, local database to search all Hackage libraries. – Petr Nov 22 '12 at 18:20

John Wiegley · Answer 3 · 2016-04-30T00:46:20.620

14

This can be done easily with lens:

import Control.Lens

over _Left (+1) $ Left 10   => Left 11
over _Left (+1) $ Right 10  => Right 10
over _Right (+1) $ Right 10 => Right 11

edited Apr 30 '16 at 00:46

answered Nov 22 '12 at 05:45

John Wiegley

6,340
1
12
19

were `_Left` and `_Right` spelled with lower `l` and `r` in 2012? – Qrilka Apr 28 '16 at 14:40
According to the current documentation, they are `_Left` and `_Right`. – John Wiegley Apr 30 '16 at 00:46

score 3 · Answer 4 · answered Jun 07 '18 at 17:54

3

Another simple option is mapLeft in Data.Either.Combinators:

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

answered Jun 07 '18 at 17:54

Xiaokui Shu

372
1
8

Mapping over Either's Left

4 Answers4

Linked