9. Lists

safeTail :: [a] -> Maybe [a]
safeTail []     = Nothing
safeTail (x:[]) = Nothing
safeTail (_:xs) = Just xs

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Ranges:

• [1..10] becomes enumFromTo 1 10
• [2, 4..10] becomes enumFromThenTo 2 4 10 -> [2,4,6,8,10]
• [1..] becomes enumFrom
• [1, 3..] becomes enumFromThen
• ['t'..'z'] (so, anything enumerable)

enumFromTo a b must have a < b.

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take      :: Int         -> [a] -> [a]
drop      :: Int         -> [a] -> [a]
splitAt   :: Int         -> [a] -> ([a], [a])
takeWhile :: (a -> Bool) -> [a] -> [a]
dropWhile :: (a -> Bool) -> [a] -> [a]
elem      :: (Eq a, Foldable t) => a -> t a -> Bool

---

Comprehensions:

• [x^2 | x <- [1..10]]
• [x^2 | x <- [1..10], rem x 2 == 0]
• [x^y | x <- [1..5], y <- [2, 3]] is 1, 1, 4, 8 and so on.
• [x^y | x <- [1..5], y <- [2, 3], x^y < 200]
• [(x, y) | x <- [1, 2, 3], y <- ['a', 'b']] is (1, a), (1, b), (2, a) etc.
• [x | x <- "Three Letter Acronym", elem x ['A'..'Z']] is "TLA"

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:sprint command can indicate (somewhat) what expressions have been evaluated.

λ: let blah = enumFromTo 'a' 'z'
λ: :sprint blah
blah = _
λ: take 3 blah
"abc"
λ: sprint blah
blah = 'a' : 'b' : 'c' : _

WHNF (Weak Head Normal Form) – evaluated to at least a data constructor.

• (1, 2) WHNF & NF
• (1, 1 + 1) WHNF (not NF)

length is strict in the spine but not the values. Using _ in pattern matching can be used to ignore values.

take 2 $ map (+1) [1, 2, undefined] prints [2, 3].

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From Data.Bool:

• bool :: a -> a -> Bool -> a

From Data.Char:

• chr :: Int -> Char
• ord :: Char -> Int

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Surprising import issue: do not forget that type names and constructor names can be identical. Importing a type does not automatically import the type's constructors.

import Data.Monoid -- everything, including `Any`
import Data.Monoid (Any) -- imports the Any **TYPE** (but not constructor(s)!)
import Data.Monoid (Any(..)) -- imports the Any type with all of its constructors
import Data.Monoid (Any(Any)) -- imports the Any type with just the Any constructor