Base Prelude

https://hackage.haskell.org/package/base/docs/Prelude.html
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Base Prelude is the standard Haskell module imported by default into all modules, providing core...
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### all examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Examples of the all function with a predicate, including an infinite list that hangs.

```haskell
>>> all (> 3) []
True
```

```haskell
>>> all (> 3) [1,2]
False
```

```haskell
>>> all (> 3) [1,2,3,4,5]
False
```

```haskell
>>> all (> 3) [1..]
False
```

```haskell
>>> all (> 3) [4..]
* Hangs forever *
```

--------------------------------

### unwords examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Interactive examples for the unwords function, showing that it joins words with spaces and is not an inverse of words, plus examples with empty strings.

```haskell
>>> **words (unwords [" "])
**[]
>>> **unwords (words "foo\nbar")
**"foo bar"

```

```haskell
>>> **unwords ["Lorem", "ipsum", "dolor"]
**"Lorem ipsum dolor"

```

```haskell
>>> **unwords ["foo", "bar", "", "baz"]
**"foo bar  baz"

```

--------------------------------

### scanl definition and examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Definition and examples of scanl, including a lambda example and an infinite list.

```haskell
scanl f z [x1, x2, ...] == [z, z `f` x1, (z `f` x1) `f` x2, ...]
```

```haskell
last (scanl f z xs) == foldl f z xs
```

```haskell
>>> scanl (+) 0 [1..4]
[0,1,3,6,10]
```

```haskell
>>> scanl (+) 42 []
[42]
```

```haskell
>>> scanl (-) 100 [1..4]
[100,99,97,94,90]
```

```haskell
>>> scanl (\reversedString nextChar -> nextChar : reversedString) "foo" ['a', 'b', 'c', 'd']
["foo","afoo","bafoo","cbafoo","dcbafoo"]
```

```haskell
>>> take 10 (scanl (+) 0 [1..])
[0,1,3,6,10,15,21,28,36,45]
```

```haskell
>>> take 1 (scanl undefined 'a' undefined)
"a"
```

--------------------------------

### iterate examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Examples of iterate with different functions. The last example shows that iterate id is equivalent to repeat.

```haskell
>>> **take 10 $ iterate not True
**[True,False,True,False,True,False,True,False,True,False]
```

```haskell
>>> **take 10 $ iterate (+3) 42
**[42,45,48,51,54,57,60,63,66,69]
```

```haskell
>>> **take 10 $ iterate id 1
**[1,1,1,1,1,1,1,1,1,1]
```

--------------------------------

### any examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Examples of the any function with a predicate, including an infinite list that hangs.

```haskell
>>> any (> 3) []
False
```

```haskell
>>> any (> 3) [1,2]
False
```

```haskell
>>> any (> 3) [1,2,3,4,5]
True
```

```haskell
>>> any (> 3) [1..]
True
```

```haskell
>>> any (> 3) [0, -1..]
* Hangs forever *
```

--------------------------------

### (++) example with two lists

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of appending two finite lists.

```haskell
>>> **[1, 2, 3] ++ [4, 5, 6]
**[1,2,3,4,5,6]
```

--------------------------------

### unzip3 examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Interactive examples for the unzip3 function, showing the result for an empty list and a non-empty list of triples.

```haskell
>>> **unzip3 []
**([],[],[])

```

```haskell
>>> **unzip3 [(1, 'a', True), (2, 'b', False)]
**([1,2],"ab",[True,False])

```

--------------------------------

### readFile example: read a file

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Reads a file lazily and returns its contents as a string. The example shows reading a file named "~/hello_world" and getting the string "Greetings!". This operation may fail with the same errors as `hGetContents` and `openFile`.

```haskell
>>> **readFile "~/hello_world"
**"Greetings!"

```

--------------------------------

### concat examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Examples of the concat function on containers of lists.

```haskell
>>> concat (Just [1, 2, 3])
[1,2,3]
```

```haskell
>>> concat (Left 42)
[]
```

```haskell
>>> concat [[1, 2, 3], [4, 5], [6], []]
[1,2,3,4,5,6]
```

--------------------------------

### words examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Interactive examples for the words function, showing how it splits on whitespace and trims leading/trailing spaces.

```haskell
>>> **words "Lorem ipsum\ndolor"
**["Lorem","ipsum","dolor"]

```

```haskell
>>> **words " foo bar "
**["foo","bar"]

```

--------------------------------

### and examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Examples of the and function on Bools, including an infinite list that hangs.

```haskell
>>> and []
True
```

```haskell
>>> and [True]
True
```

```haskell
>>> and [False]
False
```

```haskell
>>> and [True, True, False]
False
```

```haskell
>>> and (False : repeat True) -- Infinite list [False,True,True,True,...
False
```

```haskell
>>> and (repeat True)
* Hangs forever *
```

--------------------------------

### unzip examples: empty and non-empty lists

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Examples of `unzip` on an empty list and a list of pairs, returning a pair of lists.

```haskell
>>> unzip []
([],[])
```

```haskell
>>> unzip [(1, 'a'), (2, 'b')]
([1,2],"ab")
```

--------------------------------

### take examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Examples of take: returns the prefix of length n, or the whole list if n is larger. Negative or zero n yields an empty list.

```haskell
>>> **take 5 "Hello World!"
**"Hello"
```

```haskell
>>> **take 3 [1,2,3,4,5]
**[1,2,3]
```

```haskell
>>> **take 3 [1,2]
**[1,2]
```

```haskell
>>> **take 3 []
**[]
```

```haskell
>>> **take (-1) [1,2]
**[]
```

```haskell
>>> **take 0 [1,2]
**[]
```

--------------------------------

### repeat examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Examples of repeat: creates an infinite list with the same value. The second example shows that evaluating repeat undefined throws an exception.

```haskell
>>> **take 10 $ repeat 17
**[17,17,17,17,17,17,17,17,17, 17]
```

```haskell
>>> **repeat undefined
**[*** Exception: Prelude.undefined
```

--------------------------------

### head example on empty list (exception)

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of head on an empty list, which throws an exception because head is partial.

```haskell
>>> **head []
***** Exception: Prelude.head: empty list
```

--------------------------------

### cycle examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Examples of cycle: ties a finite list into a circular one. Empty list throws an exception; the last example shows laziness.

```haskell
>>> **cycle []
***** Exception: Prelude.cycle: empty list
```

```haskell
>>> **take 10 (cycle [42])
**[42,42,42,42,42,42,42,42,42,42]
```

```haskell
>>> **take 10 (cycle [2, 5, 7])
**[2,5,7,2,5,7,2,5,7,2]
```

```haskell
>>> **take 1 (cycle (42 : undefined))
**[42]
```

--------------------------------

### (++) example with empty first list

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of appending an empty list to a non-empty list.

```haskell
>>> **[] ++ [1, 2, 3]
**[1,2,3]
```

--------------------------------

### or examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Examples of the or function on Bools, including an infinite list that hangs.

```haskell
>>> or []
False
```

```haskell
>>> or [True]
True
```

```haskell
>>> or [False]
False
```

```haskell
>>> or [True, True, False]
True
```

```haskell
>>> or (True : repeat False) -- Infinite list [True,False,False,False,...
True
```

```haskell
>>> or (repeat False)
* Hangs forever *
```

--------------------------------

### concatMap examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Examples of the concatMap function mapping and concatenating lists.

```haskell
>>> concatMap (take 3) [[1..], [10..], [100..], [1000..]]
[1,2,3,10,11,12,100,101,102,1000,1001,1002]
```

```haskell
>>> concatMap (take 3) (Just [1..])
[1,2,3]
```

--------------------------------

### replicate examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Examples of replicate: returns a list of length n with the same value. Negative n yields an empty list.

```haskell
>>> **replicate 0 True
**[]
```

```haskell
>>> **replicate (-1) True
**[]
```

```haskell
>>> **replicate 4 True
**[True,True,True,True]
```

--------------------------------

### tail example on empty list (exception)

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of tail on an empty list, which throws an exception because tail is partial.

```haskell
>>> **tail []
***** Exception: Prelude.tail: empty list
```

--------------------------------

### sequenceA examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Examples for the sequenceA method of the Traversable class, showing full evaluation and short-circuiting with Nothing and Left.

```haskell
>>> **sequenceA [Just 1, Just 2, Just 3]
**Just [1,2,3]
```

```haskell
>>> **sequenceA [Right 1, Right 2, Right 3]
**Right [1,2,3]
```

```haskell
>>> **sequenceA [Just 1, Just 2, Just 3, Nothing]
**Nothing
```

```haskell
>>> **sequenceA [Right 1, Right 2, Right 3, Left 4]
**Left 4
```

--------------------------------

### (++) example with empty second list

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of appending an empty list to a non-empty list.

```haskell
>>> **[3, 2, 1] ++ []
**[3,2,1]
```

--------------------------------

### head example on non-empty list

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of head returning the first element of a non-empty list.

```haskell
>>> **head [1, 2, 3]
**1
```

--------------------------------

### mconcat example

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

GHCi example showing mconcat folding a list of strings into a single string.

```haskell
>>> mconcat ["Hello", " ", "Haskell", "!"]
"Hello Haskell!"
```

--------------------------------

### head example on infinite list

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of head on an infinite list, showing it works because it only needs the first element.

```haskell
>>> **head [1..]
**1
```

--------------------------------

### (.) example: map with composition

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of using composition to double the length of each list. The output is [0,6,2].

```haskell
>>> **map ((*2) . length) [[], [0, 1, 2], [0]]
**[0,6,2]
```

--------------------------------

### reverse examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Examples of the reverse function on lists, including an infinite list that hangs.

```haskell
>>> reverse []
[]
```

```haskell
>>> reverse [42]
[42]
```

```haskell
>>> reverse [2,5,7]
[7,5,2]
```

```haskell
>>> reverse [1..]
* Hangs forever *
```

--------------------------------

### ($) example: section

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of using ($) as a section to apply a value to a list of functions. The output is [6, 32].

```haskell
applyFive :: [Int]
applyFive = map ($ 5) [(+1), (2^)]
>>> [6, 32]
```

--------------------------------

### last example on empty list (exception)

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of last on an empty list, which throws an exception because last is partial.

```haskell
>>> **last []
***** Exception: Prelude.last: empty list
```

--------------------------------

### curry fst example

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Interactive GHCi example showing that curry fst applied to 1 and 2 yields 1.

```haskell
>>> **curry fst 1 2
**1

```

--------------------------------

### drop examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Examples of drop: returns the suffix after the first n elements, or [] if n is larger. Negative n returns the whole list.

```haskell
>>> **drop 6 "Hello World!"
**"World!"
```

```haskell
>>> **drop 3 [1,2,3,4,5]
**[4,5]
```

```haskell
>>> **drop 3 [1,2]
**[]
```

```haskell
>>> **drop 3 []
**[]
```

```haskell
>>> **drop (-1) [1,2]
**[1,2]
```

```haskell
>>> **drop 0 [1,2]
**[1,2]
```

--------------------------------

### foldMap basic usage examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Basic usage examples of foldMap with Sum, Product, and replicate, showing the resulting monoid values.

```haskell
>>> **foldMap Sum [1, 3, 5]
**Sum {getSum = 9}
```

```haskell
>>> **foldMap Product [1, 3, 5]
**Product {getProduct = 15}
```

```haskell
>>> **foldMap (replicate 3) [1, 2, 3]
**[1,1,1,2,2,2,3,3,3]
```

--------------------------------

### tail example on non-empty list

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of tail returning the list without its first element.

```haskell
>>> **tail [1, 2, 3]
**[2,3]
```

--------------------------------

### tail example on single-element list

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of tail on a single-element list, returning an empty list.

```haskell
>>> **tail [1]
**[]
```

--------------------------------

### (.) example: foldr with composition

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of folding a list of functions with composition and id as the base. The output is 25.

```haskell
>>> **foldr (.) id [(+1), (*3), (^3)] 2
**25
```

--------------------------------

### (.) example: composing composition

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of composing composition operators to create a custom operator. The output is 30.

```haskell
>>> **let (...) = (.).(.) in ((*2)...(+)) 5 10
**30
```

--------------------------------

### const example: basic usage

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of const returning the first argument and ignoring the second. The output is 42.

```haskell
>>> **const 42 "hello"
**42
```

--------------------------------

### uncurry (+) example

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Interactive GHCi example showing that uncurry (+) applied to the pair (1,2) yields 3.

```haskell
>>> **uncurry (+) (1,2)
**3

```

--------------------------------

### map (uncurry max) example

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Interactive GHCi example showing that mapping uncurry max over a list of pairs yields the list [2,4,8].

```haskell
>>> **map (uncurry max) [(1,2), (3,4), (6,8)]
**[2,4,8]

```

--------------------------------

### traverse examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Examples for the traverse method of the Traversable class, showing successful collection and short-circuiting with Nothing and Left.

```haskell
>>> **traverse Just [1,2,3,4]
**Just [1,2,3,4]
```

```haskell
>>> **traverse id [Right 1, Right 2, Right 3, Right 4]
**Right [1,2,3,4]
```

```haskell
>>> **traverse (const Nothing) [1,2,3,4]
**Nothing
```

```haskell
>>> **traverse (\x -> if odd x then Just x else Nothing)  [1,2,3,4]
**Nothing
```

```haskell
>>> **traverse id [Right 1, Right 2, Right 3, Right 4, Left 0]
**Left 0
```

--------------------------------

### unlines examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Interactive examples for the unlines function, showing that it appends a newline to each string and concatenates, and that unlines . lines is not the identity on unterminated input.

```haskell
>>> **unlines ["Hello", "World", "!"]
**"Hello\nWorld\n!\n"

```

```haskell
>>> **unlines . lines $ "foo\nbar"
**"foo\nbar\n"

```

--------------------------------

### init examples: non-empty, singleton, and empty list

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Shows the behavior of the partial function init on a non-empty list, a singleton list, and an empty list. The empty list case raises the exception Prelude.init: empty list.

```haskell
>>> **init [1, 2, 3]
**[1,2]
```

```haskell
>>> **init [1]
**[]
```

```haskell
>>> **init []
***** Exception: Prelude.init: empty list
```

--------------------------------

### last example on infinite list (hangs)

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of last on an infinite list, which hangs forever because it must traverse the entire list.

```haskell
>>> **last [1..]
*** Hangs forever *
```

--------------------------------

### uncurry ($) example

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Interactive GHCi example showing that uncurry ($) applied to the pair (show, 1) yields the string "1".

```haskell
>>> **uncurry ($) (show, 1)
**"1"

```

--------------------------------

### product basic usage examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Basic usage of product with various numeric inputs. An infinite list causes the computation to hang.

```haskell
>>> **product []
**1
```

```haskell
>>> **product [42]
**42
```

```haskell
>>> **product [1..10]
**3628800
```

```haskell
>>> **product [4.1, 2.0, 1.7]
**13.939999999999998
```

```haskell
>>> **product [1..]
*** Hangs forever *
```

--------------------------------

### scanr definition and examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Definition and examples of scanr, including a lambda example and a stack overflow on an infinite list.

```haskell
head (scanr f z xs) == foldr f z xs.
```

```haskell
>>> scanr (+) 0 [1..4]
[10,9,7,4,0]
```

```haskell
>>> scanr (+) 42 []
[42]
```

```haskell
>>> scanr (-) 100 [1..4]
[98,-97,99,-96,100]
```

```haskell
>>> scanr (\nextChar reversedString -> nextChar : reversedString) "foo" ['a', 'b', 'c', 'd']
["abcdfoo","bcdfoo","cdfoo","dfoo","foo"]
```

```haskell
>>> force $ scanr (+) 0 [1..]
* Exception: stack overflow
```

--------------------------------

### splitAt examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Interactive GHCi examples showing the behavior of `splitAt` with various indices and lists. They illustrate splitting strings and lists, handling indices larger than the list length, and negative indices.

```haskell
>>> **splitAt 6 "Hello World!"
**("Hello ","World!")
```

```haskell
>>> **splitAt 3 [1,2,3,4,5]
**([1,2,3],[4,5])
```

```haskell
>>> **splitAt 1 [1,2,3]
**([1],[2,3])
```

```haskell
>>> **splitAt 3 [1,2,3]
**([1,2,3],[])
```

```haskell
>>> **splitAt 4 [1,2,3]
**([1,2,3],[])
```

```haskell
>>> **splitAt 0 [1,2,3]
**([],[1,2,3])
```

```haskell
>>> **splitAt (-1) [1,2,3]
**([],[1,2,3])
```

--------------------------------

### lines examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Interactive examples for the lines function, showing how it splits strings on newline characters, including edge cases for empty input, trailing newlines, and unterminated final lines.

```haskell
>>> **lines ""           -- empty input contains no lines
**[]

```

```haskell
>>> **lines "\n"         -- single empty line
**[""]

```

```haskell
>>> **lines "one"        -- single unterminated line
**["one"]

```

```haskell
>>> **lines "one\n"      -- single non-empty line
**["one"]

```

```haskell
>>> **lines "one\n\n"    -- second line is empty
**["one",""]

```

```haskell
>>> **lines "one\ntwo"   -- second line is unterminated
**["one","two"]

```

```haskell
>>> **lines "one\ntwo\n" -- two non-empty lines
**["one","two"]

```

--------------------------------

### appendFile example: write, append, and read

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Writes "hello" to a file, appends " world!", then reads and prints the result, yielding "hello world!". The example notes that this could be more efficiently written by acquiring a handle with `openFile` and using handle-writing computations such as `hPutStr`.

```haskell
>>> **let fn = "hello_world"
**>>> **in writeFile fn "hello" >> appendFile fn " world!" >> (readFile fn >>= putStrLn)
**"hello world!"

```

--------------------------------

### scanr1 examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Examples of scanr1 with different operators and edge cases. The last example shows that forcing an infinite list with scanr1 causes a stack overflow.

```haskell
>>> **scanr1 (+) [1..4]
**[10,9,7,4]
```

```haskell
>>> **scanr1 (+) []
**[]
```

```haskell
>>> **scanr1 (-) [1..4]
**[-2,3,-1,4]
```

```haskell
>>> **scanr1 (&&) [True, False, True, True]
**[False,False,True,True]
```

```haskell
>>> **scanr1 (||) [True, True, False, False]
**[True,True,False,False]
```

```haskell
>>> **force $ scanr1 (+) [1..]
***** Exception: stack overflow
```

--------------------------------

### IO action sequencing with <>

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of the (<>) operator for IO actions, sequencing two output actions.

```haskell
>>> putStr "Hello, " <> putStrLn "World!"
Hello, World!
```

--------------------------------

### readFile example: read first five bytes

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Reads the first five bytes of a file using `readFile` and `take`. The example reads from `/dev/zero` and returns a string of five NUL characters. `readFile` reads lazily, on demand, as with `getContents`.

```haskell
>>> **take 5 <$> readFile "/dev/zero"
**"\NUL\NUL\NUL\NUL\NUL"

```

--------------------------------

### filter odd example

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of filter keeping only odd numbers.

```haskell
>>> **filter odd [1, 2, 3]
**[1,3]
```

--------------------------------

### getLine example

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Demonstrates getLine reading a line from stdin. The input 'Hello World!' is echoed and the result is the string without the newline.

```haskell
>>> getLine
**> Hello World!
"Hello World!"
```

--------------------------------

### filter (/= 3) example

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of filter removing all occurrences of 3.

```haskell
>>> **filter (/= 3) [1, 2, 3, 4, 3, 2, 1]
**[1,2,4,2,1]
```

--------------------------------

### map (+1) example

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of map adding 1 to each element of a list.

```haskell
>>> **map (+1) [1, 2, 3]
**[2,3,4]
```

--------------------------------

### ($) example: with operator

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of the same function using ($) to avoid parentheses. This is the version with the operator.

```haskell
-- | Sum numbers in a string: strSum "100  5 -7" == 98
strSum :: String -> Int
strSum s = sum $ mapMaybe readMaybe $ words s
```

--------------------------------

### takeWhile examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Examples of takeWhile: returns the longest prefix of elements satisfying the predicate.

```haskell
>>> **takeWhile (< 3) [1,2,3,4,1,2,3,4]
**[1,2]
```

```haskell
>>> **takeWhile (< 9) [1,2,3]
**[1,2,3]
```

```haskell
>>> **takeWhile (< 0) [1,2,3]
**[]
```

--------------------------------

### scanl1 definition and examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Definition and examples of scanl1, including boolean operators and an infinite list.

```haskell
scanl1 f [x1, x2, ...] == [x1, x1 `f` x2, ...]
```

```haskell
>>> scanl1 (+) [1..4]
[1,3,6,10]
```

```haskell
>>> scanl1 (+) []
[]
```

```haskell
>>> scanl1 (-) [1..4]
[1,-1,-4,-8]
```

```haskell
>>> scanl1 (&&) [True, False, True, True]
[True,False,False,False]
```

```haskell
>>> scanl1 (||) [False, False, True, True]
[False,False,True,True]
```

```haskell
>>> take 10 (scanl1 (+) [1..])
[1,3,6,10,15,21,28,36,45,55]
```

```haskell
>>> take 1 (scanl1 undefined ('a' : undefined))
"a"
```

--------------------------------

### show :: a -> String

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

A specialised variant of showsPrec, using precedence context zero, and returning an ordinary String.

```APIDOC
## show :: a -> String

### Description
A specialised variant of `showsPrec`, using precedence context zero, and returning an ordinary `String`.

### Method
show

### Signature
show :: a -> String

### Parameters
- **a** (type variable) - The value to convert to a String.

### Return Value
- **String** - The string representation of the value.

```

--------------------------------

### read function examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Interactive examples showing the behavior of the read function. The first parses a valid integer string, while the second demonstrates the error thrown when parsing fails.

```haskell
>>> **read "123" :: Int
**123

```

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

```

--------------------------------

### map with lambda example

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of map with a lambda expression that multiplies by 3 and adds 1.

```haskell
>>> **map (\n -> 3 * n + 1) [1, 2, 3]
**[4,7,10]
```

--------------------------------

### readLn example: read and map

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Reads a line with `readLn`, parses it, and adds 5. The example shows input `25` and output `30`. `readLn` combines `getLine` and `readIO` and may fail with the same errors as `getLine` and `readIO`.

```haskell
>>> **fmap (+ 5) readLn
**> 25
30

```

--------------------------------

### writeFile example: write and read a file

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Writes the string "world" to the file "hello" and then reads it back, returning "world". `writeFile` may fail with the same errors as `hPutStr` and `withFile`.

```haskell
>>> **writeFile "hello" "world" >> readFile "hello"
**"world"

```

--------------------------------

### dropWhile examples

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Examples of dropWhile: returns the suffix after removing the longest prefix of elements satisfying the predicate.

```haskell
>>> **dropWhile (< 3) [1,2,3,4,5,1,2,3]
**[3,4,5,1,2,3]
```

```haskell
>>> **dropWhile (< 9) [1,2,3]
**[]
```

```haskell
>>> **dropWhile (< 0) [1,2,3]
**[1,2,3]
```

--------------------------------

### last example on finite list

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of last returning the last element of a finite non-empty list.

```haskell
>>> **last [1, 2, 3]
**3
```

--------------------------------

### filter with length lambda example

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of filter with a lambda that checks if the length of a string is greater than 3.

```haskell
>>> **filter (\l -> length l > 3) ["Hello", ", ", "World", "!"]
**["Hello","World"]
```

--------------------------------

### map id example

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Example of map with the identity function, showing that map id equals id.

```haskell
>>> **map id [1, 2, 3]
**[1,2,3]
```

--------------------------------

### Hand-written Read instance with readPrec

Source: https://hackage.haskell.org/package/base/docs/Prelude.html

Recommended pattern for hand-written Read instances: implement readPrec and use readListPrecDefault for readListPrec.

```haskell
instance Read T where
  readPrec     = ...
  readListPrec = readListPrecDefault
```
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