Reimplementations of functions presented in versions of "base" newer than 4.6

Case analysis for the Bool type. bool a b p evaluates to a when p is False, and evaluates to b when p is True.

& is a reverse application operator. This provides notational convenience. Its precedence is one higher than that of the forward application operator $, which allows & to be nested in $.

Flipped version of <$.

The isSubsequenceOf function takes two lists and returns True if the first list is a subsequence of the second list.

isSubsequenceOf x y is equivalent to elem x (subsequences y).

Examples

>>> isSubsequenceOf "GHC" "The Glorious Haskell Compiler"
True
>>> isSubsequenceOf ['a','d'..'z'] ['a'..'z']
True
>>> isSubsequenceOf [1..10] [10,9..0]
False

Sort a list by comparing the results of a key function applied to each element. sortOn f is equivalent to sortBy . comparing f, but has the performance advantage of only evaluating f once for each element in the input list. This is called the decorate-sort-undecorate paradigm, or Schwartzian transform.

Decompose a list into its head and tail. If the list is empty, returns Nothing. If the list is non-empty, returns Just (x, xs), where x is the head of the list and xs its tail.

Like traceShow but returns the shown value instead of a third value.

Like trace but returning unit in an arbitrary monad. Allows for convenient use in do-notation. Note that the application of trace is not an action in the monad, as traceIO is in the IO monad.

... = do
  x <- ...
  traceM $ "x: " ++ show x
  y <- ...
  traceM $ "y: " ++ show y

Like traceM, but uses show on the argument to convert it to a String.

... = do
  x <- ...
  traceShowM $ x
  y <- ...
  traceShowM $ x + y