Documentation

Returns all values of some Bounded Enum in ascending order.

>>> data TrafficLight = Red | Blue | Green deriving (Show, Enum, Bounded)
>>> universe :: [TrafficLight]
[Red,Blue,Green]
>>> universe :: [Bool]
[False,True]

inverseMap f creates a function that is the inverse of a given function f. It does so by constructing Map internally for each value f a. The implementation makes sure that the Map is constructed only once and then shared for every call.

Memory usage note: don't inverse functions that have types like Int as their result. In this case the created Map will have huge size.

The complexity of reversed mapping is \(\mathcal{O}(\log n)\).

Performance note: make sure to specialize monomorphic type of your functions that use inverseMap to avoid Map reconstruction.

One of the common inverseMap use-case is inverting the show or a show-like function.

>>> data Color = Red | Green | Blue deriving (Show, Enum, Bounded)
>>> parse = inverseMap show :: String -> Maybe Color
>>> parse "Red"
Just Red
>>> parse "Black"
Nothing

Correctness note: inverseMap expects injective function as its argument, i.e. the function must map distinct arguments to distinct values.

Typical usage of this function looks like this:

data GhcVer
    = Ghc802
    | Ghc822
    | Ghc844
    | Ghc865
    | Ghc881
    deriving (Eq, Ord, Show, Enum, Bounded)

showGhcVer :: GhcVer -> Text
showGhcVer = \case
    Ghc802 -> "8.0.2"
    Ghc822 -> "8.2.2"
    Ghc844 -> "8.4.4"
    Ghc865 -> "8.6.5"
    Ghc881 -> "8.8.1"

parseGhcVer :: Text -> Maybe GhcVer
parseGhcVer = inverseMap showGhcVer

Like succ, but doesn't fail on maxBound. Instead it returns minBound.

>>> next False
True
>>> next True
False
>>> succ True
*** Exception: Prelude.Enum.Bool.succ: bad argument

Like pred, but doesn't fail on minBound. Instead it returns maxBound.

>>> prev False
True
>>> prev True
False
>>> pred False
*** Exception: Prelude.Enum.Bool.pred: bad argument

Since: 0.6.0.0

See prev.

Returns Nothing if given Int outside range.

>>> safeToEnum @Bool 0
Just False
>>> safeToEnum @Bool 1
Just True
>>> safeToEnum @Bool 2
Nothing
>>> safeToEnum @Bool (-1)
Nothing