seq package:ghc-internal

The value of seq a b is bottom if a is bottom, and otherwise equal to b. In other words, it evaluates the first argument a to weak head normal form (WHNF). seq is usually introduced to improve performance by avoiding unneeded laziness. A note on evaluation order: the expression seq a b does not guarantee that a will be evaluated before b. The only guarantee given by seq is that the both a and b will be evaluated before seq returns a value. In particular, this means that b may be evaluated before a. If you need to guarantee a specific order of evaluation, you must use the function pseq from the "parallel" package.

Evaluate each action in the sequence from left to right, and collect the results.

Evaluate each monadic action in the structure from left to right, and collect the results. For a version that ignores the results see sequence_.

Examples

Basic usage: The first two examples are instances where the input and and output of sequence are isomorphic.

>>> sequence $ Right [1,2,3,4]
[Right 1,Right 2,Right 3,Right 4]

>>> sequence $ [Right 1,Right 2,Right 3,Right 4]
Right [1,2,3,4]

The following examples demonstrate short circuit behavior for sequence.

>>> sequence $ Left [1,2,3,4]
Left [1,2,3,4]

>>> sequence $ [Left 0, Right 1,Right 2,Right 3,Right 4]
Left 0

Evaluate each monadic action in the structure from left to right, and ignore the results. For a version that doesn't ignore the results see sequence. sequence_ is just like sequenceA_, but specialised to monadic actions.

Evaluate each action in the structure from left to right, and ignore the results. For a version that doesn't ignore the results see sequenceA. sequenceA_ is just like sequence_, but generalised to Applicative actions.

Examples

Basic usage:

>>> sequenceA_ [print "Hello", print "world", print "!"]
"Hello"
"world"
"!"

Evaluate each action in the structure from left to right, and collect the results. For a version that ignores the results see sequenceA_.

Examples

Basic usage: For the first two examples we show sequenceA fully evaluating a a structure and collecting the results.

>>> sequenceA [Just 1, Just 2, Just 3]
Just [1,2,3]

>>> sequenceA [Right 1, Right 2, Right 3]
Right [1,2,3]

The next two example show Nothing and Just will short circuit the resulting structure if present in the input. For more context, check the Traversable instances for Either and Maybe.

>>> sequenceA [Just 1, Just 2, Just 3, Nothing]
Nothing

>>> sequenceA [Right 1, Right 2, Right 3, Left 4]
Left 4

The primitive used to implement evaluate. Prefer to use evaluate whenever possible!

This function is only used in Quote when desugaring brackets. This is not necessary for the user, who can use the ordinary return and (>>=) operations.

The isSubsequenceOf function takes two lists and returns True if all the elements of the first list occur, in order, in the second. The elements do not have to occur consecutively. isSubsequenceOf x y is equivalent to x `elem` (subsequences y). Note: isSubsequenceOf is often used in infix form.

Examples

>>> "GHC" `isSubsequenceOf` "The Glorious Haskell Compiler"
True

>>> ['a','d'..'z'] `isSubsequenceOf` ['a'..'z']
True

>>> [1..10] `isSubsequenceOf` [10,9..0]
False

For the result to be True, the first list must be finite; for the result to be False, the second list must be finite:

>>> [0,2..10] `isSubsequenceOf` [0..]
True

>>> [0..] `isSubsequenceOf` [0,2..10]
False

>>> [0,2..] `isSubsequenceOf` [0..]
* Hangs forever*

The subsequences function returns the list of all subsequences of the argument.

Laziness

subsequences does not look ahead unless it must:

>>> take 1 (subsequences undefined)
[[]]

>>> take 2 (subsequences ('a' : undefined))
["","a"]

Examples

>>> subsequences "abc"
["","a","b","ab","c","ac","bc","abc"]

This function is productive on infinite inputs:

>>> take 8 $ subsequences ['a'..]
["","a","b","ab","c","ac","bc","abc"]

Stopped because there are sufficient free elements in the output to output at least one encoded ASCII character, but the input contains an invalid or unrepresentable sequence

{ [ 1 ,2 .. 10 ] }