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Mutable variable

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Although the functional programming paradigm emphasises the virtues of immutable variables, sometimes you need mutable variables nonetheless.

You can either simulate mutable variables using the state monad provided for instance by
Control.Monad.Trans.State
in the transformers package or you can use really mutable variables as provided by
Data.IORef
or
Data.STRef
or
Control.Concurrent.STM.TVar
from the stm package. In either case you need a monad in order to cope with mutability, while staying purely functional.

There are several packages that provide a single interface to these different implementations of mutable variables (in alphabetical order):

See also