65 lines
1.3 KiB
Markdown
65 lines
1.3 KiB
Markdown
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---
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title: Lists
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---
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Lists are a widely used datatype in Haskell. In fact, if you have used strings you've used Haskell's lists!
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# Definition
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Haskell's lists are recursively defined as follows:
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```haskell
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data [] a -- A List containing type `a`
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= [] -- Empty list constructor.
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| a : [a] -- "Construction" constructor, a.k.a. cons.
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```
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Notice that lists in Haskell are not arrays, but linked lists.
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The following are examples of lists:
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```haskell
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empty :: [()]
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empty = []
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ints :: [Int]
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ints = 1 : 2 : 3 : []
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```
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There's syntactic sugar for making lists as well:
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```haskell
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bools :: [Bool]
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bools = [True, False, False, True] -- True : False : False : True : []
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```
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`String` is just an alias for `[Char]`!
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```haskell
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chars :: [Char]
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chars = "This is a character list!"
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```
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# Functions
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Lists have many different built in functions. Here's a few:
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```haskell
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-- Concatenation:
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-- Stick two lists together.
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greeting :: String
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greeting = "Hello, " ++ "World!" -- "Hello, World!"
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-- Map:
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-- Appy some function to overy element.
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abc :: [Int]
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abc = map succ [0, 1, 2] -- [1, 2, 3]
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```
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# Pattern matching
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You can easily pattern match lists to easily recurse over them.
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```haskell
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map' :: (a -> b) -> [a] -> [b]
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map' _ [] = [] -- Base case.
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map' f (a:as) = f a : map' f as -- Recursive case.
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```
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