Module

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.

#Ref 

data Ref t0

A value of type Ref a represents a mutable reference which holds a value of type a.

#new 

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

#newWithSelf 

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

Create a new mutable reference containing a value that can refer to the Ref being created.

#read 

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

Read the current value of a mutable reference.

#modify' 

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

Update the value of a mutable reference by applying a function to the current value.

#modify 

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

Update the value of a mutable reference by applying a function to the current value. The updated value is returned.

#modify_ 

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

A version of modify which does not return the updated value.

#write 

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

Update the value of a mutable reference to the specified value.

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