Data.Supply

The technique for generating new values is based on the paper ''On Generating Unique Names'' by Lennart Augustsson, Mikael Rittri, and Dan Synek.

Creating supplies

data Supply a

A type that can be used to generate values on demand.

Instances

Functor Supply

newSupply :: a -> (a -> a) -> IO (Supply a)

Creates a new supply of values. The arguments specify how to generate values: the first argument is an initial value, the second specifies how to generate a new value from an existing one.

newEnumSupply :: Enum a => IO (Supply a)

A supply of values that are in the Enum class. The initial value is toEnum 0, new values are generates with succ.

newNumSupply :: Num a => IO (Supply a)

A supply of values that are in the Num class. The initial value is 0, new values are generated by adding 1.

newDupableSupply :: a -> (a -> a) -> IO (Supply a)

Create a new supply of values. WARNING: This version is faster then newSupply but it is not completely thread safe, so use only if performance is an issue!

Rules for using the generated supplies: * Supply splitting should be evaluated in a single thread. For example, use case with split2 to force the splitting of a supply. * Different threads should work with different supplies. For example, one could (strictly) split a supply, and then fork new threads with the resulting supplies.

newDupableEnumSupply :: Enum a => IO (Supply a)

A supply of values that are in the Enum class. The initial value is toEnum 0, new values are generates with succ. WARNING: See comment on newDupableSupply

newDupableNumSupply :: Num a => IO (Supply a)

A supply of values that are in the Num class. The initial value is 0, new values are generated by adding 1. WARNING: See comment on newDupableSupply

Obtaining values from supplies

supplyValue :: Supply a -> a

Get the value of a supply. This function, together with modifySupply forms a comonad on Supply.

Generating new supplies from old

modifySupply :: Supply a -> (Supply a -> b) -> Supply b

Generate a new supply by systematically applying a function to an existing supply. This function, together with supplyValue form a comonad on Supply.

split :: Supply a -> [Supply a]

Generate an infinite list of supplies.

split2 :: Supply a -> (Supply a, Supply a)

Split a supply into two different supplies. The resulting supplies are different from the input supply.

split3 :: Supply a -> (Supply a, Supply a, Supply a)

Split a supply into three different supplies.

split4 :: Supply a -> (Supply a, Supply a, Supply a, Supply a)

Split a supply into four different supplies.