Write You a Haskell Stephen Diehl

length :: [a] -> Int
length [] = 0
length (x:xs) = 1 + (length xs)
fst :: (a, b) -> a
fst (a,b) = a
Patterns may be guarded by predicates (functions which yield a boolean). Guards only allow the execution
of a branch if the corresponding predicate yields True.
filter :: (a -> Bool) -> [a] -> [a]
filter pred [] = []
filter pred (x:xs)
| pred x = x : filter pred xs
| otherwise = filter pred xs
Recursion
In Haskell all iteration over data structures is performed by recursion. Entering a function in Haskell
does not create a new stack frame, the logic of the function is simply entered with the arguments on the
stack and yields result to the register. In the case where a function returns an invocation of itself invoked
in the tail position the resulting logic is compiled identically to while loops in other languages, via a jmp
instruction instead of a call.
sum :: [Int] -> [Int]
sum ys = go ys 0
where
go (x:xs) i = go xs (i+x)
go [] i = i
Functions can be defined to recurse mutually on each other.
even 0 = True
even n = odd (n-1)
odd 0 = False
odd n = even (n-1)
Laziness
A Haskell program can be thought of as being equivalent to a large directed graph. Each edge represents
the use of a value, and each node is the source of a value. A node can be:
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