Effect.Ref

This module defines the Ref type for mutable value references, as well as actions for working with them.

You'll notice that all of the functions that operate on a Ref (e.g. new, read, write) return their result wrapped in an Effect. Working with mutable references is considered effectful in PureScript because of the principle of purity: functions should not have side effects, and should return the same result when called with the same arguments. If a Ref could be written to without using Effect, that would cause a side effect (the effect of changing the result of subsequent reads for that Ref). If there were a function for reading the current value of a Ref without the result being wrapped in Effect, the result of calling that function would change each time a new value was written to the Ref. Even creating a new Ref is effectful: if there were a function for creating a new Ref with the type forall s. s -> Ref s, then calling that function twice with the same argument would not give the same result in each case, since you'd end up with two distinct references which could be updated independently of each other.

Note: Control.Monad.ST provides a pure alternative to Ref when mutation is restricted to a local scope.

data Ref :: Type -> Type

new :: forall s. s -> Effect (Ref s)

newWithSelf :: forall s. (Ref s -> s) -> Effect (Ref s)

read :: forall s. Ref s -> Effect s

modify' :: forall s b. (s -> { state :: s, value :: b }) -> Ref s -> Effect b

modify :: forall s. (s -> s) -> Ref s -> Effect s

modify_ :: forall s. (s -> s) -> Ref s -> Effect Unit

write :: forall s. s -> Ref s -> Effect Unit