Documentation

\(\mathcal{O}(1)\) . Extract the first element of a list, which must be non-empty.

\(\mathcal{O}(n)\) . Return all the elements of a list except the last one. The list must be non-empty.

\(\mathcal{O}(1)\) . Extract the elements after the head of a list, which must be non-empty.

\(\mathcal{O}(n)\) . Extract the last element of a list, which must be finite and non-empty.

Left-associative fold of a structure.

In the case of lists, foldl , when applied to a binary operator, a starting value (typically the left-identity of the operator), and a list, reduces the list using the binary operator, from left to right:

foldl f z [x1, x2, ..., xn] == (...((z `f` x1) `f` x2) `f`...) `f` xn

Note that to produce the outermost application of the operator the entire input list must be traversed. This means that foldl' will diverge if given an infinite list.

Also note that if you want an efficient left-fold, you probably want to use foldl' instead of foldl . The reason for this is that latter does not force the "inner" results (e.g. z `f` x1 in the above example) before applying them to the operator (e.g. to (`f` x2) ). This results in a thunk chain \(\mathcal{O}(n)\) elements long, which then must be evaluated from the outside-in.

For a general Foldable structure this should be semantically identical to,

foldl f z = foldl f z . toList

Right-associative fold of a structure.

In the case of lists, foldr , when applied to a binary operator, a starting value (typically the right-identity of the operator), and a list, reduces the list using the binary operator, from right to left:

foldr f z [x1, x2, ..., xn] == x1 `f` (x2 `f` ... (xn `f` z)...)

Note that, since the head of the resulting expression is produced by an application of the operator to the first element of the list, foldr can produce a terminating expression from an infinite list.

For a general Foldable structure this should be semantically identical to,

foldr f z = foldr f z . toList

Left-associative fold of a structure but with strict application of the operator.

This ensures that each step of the fold is forced to weak head normal form before being applied, avoiding the collection of thunks that would otherwise occur. This is often what you want to strictly reduce a finite list to a single, monolithic result (e.g. length ).

For a general Foldable structure this should be semantically identical to,

foldl' f z = foldl' f z . toList

Since: base-4.6.0.0

Right-associative fold of a structure, but with strict application of the operator.

Since: base-4.6.0.0

A variant of foldr that has no base case, and thus may only be applied to non-empty structures.

foldr1 f = foldr1 f . toList

A variant of foldl that has no base case, and thus may only be applied to non-empty structures.

foldl1 f = foldl1 f . toList

cycle ties a finite list into a circular one, or equivalently, the infinite repetition of the original list. It is the identity on infinite lists.

The largest element of a non-empty structure.

Since: base-4.8.0.0

The least element of a non-empty structure.

Since: base-4.8.0.0

List index (subscript) operator, starting from 0. It is an instance of the more general genericIndex , which takes an index of any integral type.

The sum function computes the sum of the numbers of a structure.

Since: base-4.8.0.0

The product function computes the product of the numbers of a structure.

Since: base-4.8.0.0

The fromJust function extracts the element out of a Just and throws an error if its argument is Nothing .

Examples

>>> fromJust (Just 1)
1

>>> 2 * (fromJust (Just 10))
20

>>> 2 * (fromJust Nothing)
*** Exception: Maybe.fromJust: Nothing

The read function reads input from a string, which must be completely consumed by the input process. read fails with an error if the parse is unsuccessful, and it is therefore discouraged from being used in real applications. Use readMaybe or readEither for safe alternatives.

>>> read "123" :: Int
123

>>> read "hello" :: Int
*** Exception: Prelude.read: no parse