CODEWITHSUNDEEP
haskell
greole
greole

Reputation: 4771

Difference between 'return' and 'pure'

What is the difference between return and pure from Control.Applicative ? It seems that I can use pure even at the end of a do block?

So is there any situation where one should be preferred over the other (besides that everyone expects a return at the end of a do Block)?

Upvotes: 52

Views: 8906

Answers (2)

Lee
Lee

Reputation: 144206

The Applicative typeclass was added after Monad and historically the Monad class has not been a subclass of Applicative. This was changed fairly recently in the Applicative-Monad-Proposal and this means that return a should be equivalent to pure a for every Monad instance.

There is a proposal to move return out of the Monad class and make it an alias for pure.

Therefore when dealing with a Monad you should always be able to use pure instead of return. You cannot go the other way however since pure has a more general type than return since it only returns an Applicative. For example the following

wontCheck :: Applicative f => f Int
wontCheck = return 4

won't type check since return requires f to be a Monad.

Upvotes: 31

Rein Henrichs
Rein Henrichs

Reputation: 15605

In GHC 7.8 and before, Applicative was not a superclass of Monad. It was even possible for a Monad instance to not have an Applicative instance. There was, however, an expectation that pure and return should have the same behavior for types that are instances of both.

In GHC 7.10, due to the Functor-Applicative-Monad Proposal, Applicative is now a superclass of Monad (class Applicative m => Monad m) and it is now a rule that pure and return must be the same for all Monad instances. In fact, the default implementation of return is now pure, as seen in the source on hackage.

pure might be preferred to return because it does not incur a Monad constraint, only an Applicative constraint, thus making the function more general. return might be preferred to pure in do notation because of historical precedent, but pure could be used to exactly the same effect.

Upvotes: 67

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