haskell - difference between two monads: ErrorT e Identity v and Either e v -

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can explain difference between 

errort string identity integer

and either string integer ?

(to simplify this, i'm going answer in terms of exceptt type instead of the deprecated errort. answer not strictly true errort, it's true modulo annoying facts led deprecation of errort.)

the key concept understand here isomorphism. put very informally, 2 types isomorphic when, in spite of being superficially different, "essentially same."

we can put bit more meat on notion adding concept: 2 haskell types isomorphic if both can converted other in "lossless" fashion pair of inverse functions. in case, either e a , eithert e identity a isomorphic because following 2 functions inverses:

toeither :: exceptt e identity -> either e toeither ma = runidentity (runexceptt ma)  toexceptt :: either e -> exceptt e identity toexceptt (left e) = throwerror e toexceptt (right a) = return

so going informal remarks above, tells 2 types "essentially same." , meat inverse functions add prove that:

  1. if have piece of code uses either type, can refactor use other , code produce same results , behavior;
  2. if have 2 separate libraries, 1 of them uses first type , other uses second, can bridge between them using isomorphism functions , fine.

so comes down in programming there many different ways of doing exactly same thing. in case, exceptt more general version of either, when plug in identity base monad works same either does. prefer use either in case because don't have boilerplate, still find ourselves in situation exceptt e identity a , knowing isomorphism helps understand it's not different either e a.

here's way analyze this. exceptt type defined this:

newtype exceptt e m = exceptt { runexceptt :: m (either e a) }

in haskell, newtype definition isomorphism (the exceptt constructor , runexceptt function inverses), in case means following 2 types isomorphic:

exceptt e m ~ m (either e a)

which means these 2 isomorphic well:

exceptt e identity ~ identity (either e a)

but identity defined newtype:

newtype identity = identity { runidentity :: }

which means isomorphism holds well:

identity ~

and therefore these ones:

identity (either e a) ~ either e exceptt e identity ~ either e

so when library transparently defines type newtype, pays notice fact. (and that's why library documentation in haskell tells whether data types exported constructors data or newtype definition—knowing it's newtype big deal.)

another benefit of looking @ in terms of isomorphisms helps understand lot of standard haskell libraries better. said above, newtype definition of exceptt, following isomorphism holds

exceptt e m ~ m (either e a)

when working monadic type of form m a, call m "monad" , a "result type". looking @ through lens, then:

  1. in m (either e a), m monad , either e a result type;
  2. in exceptt e m a, exceptt e m monad , a result type.

so exceptt monad transformer isomorphism allows "flip perspective" on computations of type m (either e a) treat a result type instead of either e a. "changing meaning" of monad operations mean in m accounts "perspective flip" in question.


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