So the general pattern is:-- See the section on functions for information on how to write your own.---------------------------------------------------------------------------------------------------------- Every element in a list must have the same type.-- Infinite lists work because Haskell has "lazy evaluation". Haskell makes coding a real joy for me.There’s a lot more to Haskell, including typeclasses and monads. However, mapping can be expressed much more concisely with In Python, the debate is between immutable list comprehensions and mutating loops. Like other languages, Haskell does have its own functional definition and declaration.
(map f . big ideas that make Haskell such fun to code in. Lists are a fundamental part of Haskell, and we've used them extensively before getting to this chapter. Any kind of join of xs and ys).For list comprehensions versus monadic do notation: If what you trying to express is basically math, then list comprehensions seem a much more natural way to do that - and more declarative.map is used a lot (more precisely, fmap, which is a generalization to a broader set of types beyond list) because it fits with some of the language machinery around types and typeclasses.However list comprehensions work almost the same as python (in fact I think the python implementation came from haskell although I don't know the whole history), so if you can express yourself eloquently with list comprehensions, you should go ahead and use them.But eventually though, learn the ideas behind haskell semantics, don't just transplant 1-1 operations from other languages. Like map, a fold is a higher order function that takes a function and a list. Such a low priority means that-- the expression on its right is applied as a parameter to the function on its left.---------------------------------------------------------------------------------------------------------- Haskell has a very strong type system, and every valid expression has a type.-- When you define a value, it's good practice to write its type above it:---------------------------------------------------------------------------------------------------------- if-expressions can be on multiple lines too, indentation is important-- case expressions: Here's how you could parse command line arguments-- Haskell doesn't have loops; it uses recursion instead.-- map applies a function over every element in a list---------------------------------------------------------------------------------------------------------- A data type is declared with a 'type constructor' on the left-- and one or more 'data constructors' on the right, separated by-- the pipe | symbol. These are the
Any new values-- You can see the type of any value or expression with `:t`:-- Operators, such as `+`, `:` and `$`, are functions.-- Their type can be inspected by putting the operator in parentheses:-- You can get additional information on any `name` using `:i`: ... (map f xs). map g actually never generates an intermediate list -- the final list is generated directly from the first list, because of how lazy lists work: map f [] = [] map f (x:xs) = f x : map … O(n*log n).map f s is the set obtained by applying f to each element of s.It's worth noting that the size of the result may be smaller if, for some (x,y), x /= y && f x == f y The map function is polymorphic and its type indicates clearly that its first argument is a function; note also that the two a's must be instantiated with the same type (likewise for the b's). Problem Solution Examples In contrast to standard function application, which-- has highest possible priority of 10 and is left-associative, the `$` operator-- has priority of 0 and is right-associative.
This operator applies a function-- to a given parameter. Let's look at a few concrete examples. This is a sum/union type. Functions play a major role in Haskell, as it is a functional programming language.
The way Haskell uses a monad to-- do IO allows it to be a purely functional language. map g) isn't slower than list comprehensions, actually. List comprehensions. The novel insight is that the list type is a monad too! We can also-- The type of the `do` statement is that of its last line.-- `return` is not a keyword, but merely a function-- The type `IO` is an example of a "monad". In Haskell, lists are what Arrays are in most other languages. So you can ask for-- the 1000th element of your list and Haskell will give it to you:-- And now Haskell has evaluated elements 1 - 1000 of this list...but the-- rest of the elements of this "infinite" list don't exist yet! We can write-- Anonymous functions are created with a backslash followed by-- using fold (called `inject` in some languages) with an anonymous-- function.