Documentation

The typeReps function retrieves all the type representations found while traversing the data given as parameter.

CAUTION: for newtypes, do not use 'deriving newtype (HasTypeReps)' to derive instances, rather use 'deriving anyclass (HasTypeReps)'. This is because we use these instances in abstractSize, and for that we prefer to have the newtype wrapper type available for "costing". The difference between 'newtype' and anyclass instances is as follows:

newtype Hash = Hash { unHash :: Int } deriving newtype (..., HasTypeReps) > typeReps someHash = Seq.fromList [Int] vs newtype Hash = Hash { unHash :: Int } deriving stock (...,Generics); deriving anyclass (HasTypeReps) > typeReps someHash = Seq.fromList [Hash, Int]

Examples:

>>> typeReps "a"
fromList [[Char],Char]

>>> typeReps "ab"
fromList [[Char],Char,Char]

>>> typeReps ([] :: [Int])
fromList [[Int]]

>>> :set -XDeriveGeneric
>>> data Foo = Foo [Int] (Char, Char) deriving (Generic)
>>> instance HasTypeReps Foo
>>> typeReps $ Foo [1, 2] ('a', 'b')
fromList [Foo,[Int],Int,Int,(Char,Char),Char,Char]

abstractSize m a computes the abstract size of a, using the accounting map m. The map m determines the abstract size of each TypeRep contained in a, and this function simply adds all the individual abstract sizes. To be able to extract the type representations (TypeReps) inside a, we require it to be an instance of HasTypeReps.

Examples:

>>> :set -XOverloadedLists
>>> abstractSize [(typeOf (undefined:: Char), 10)] 'a'
10

>>> abstractSize [(typeOf 'x', 10)] "hello"
50

>>> abstractSize [(typeOf 'x', 10), (typeOf True, 100)] ("hello", False)
150

>>> abstractSize [(typeOf (undefined :: [Int]), 6), (typeOf (1 :: Int), 1)] ([0, 1, 2, 3] :: [Int])
10

>>> abstractSize [(typeOf (undefined :: [Int]), 3), (typeOf (1 :: Int), -1)] ([0, 1, 2] :: [Int])
0