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. A supply may be turned into two different supplies by using the functions supplyLeft and supplyRight.

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.

unsafeNewIntSupply :: IO (Supply Int)

Create a supply of ints. WARNING: In general, this is not thread safe! It should be OK, as long as the supply is not accessed by different threads. So, if you are in a multi-threaded setting, first split the supply, and give different supply values to the different threads.

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

supplyLeft :: Supply a -> Supply a

Generate a new supply. This supply is different from the one generated with supplyRight.

supplyRight :: Supply a -> Supply a

Generate a new supply. This supply is different from the one generated with supplyLeft.

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 by using supplyLeft and supplyRight repeatedly.

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.