Uses the Haskell id function

Question

What are the id functions used in Haskell?

Answer 1

This is useful as an argument for higher order functions (functions that take functions as arguments), where you want a particular value to remain unchanged.

Example 1 Leave the value alone if it is in order, otherwise return the default value of 7.

Prelude Data.Maybe> :t maybe maybe :: b -> (a -> b) -> Maybe a -> b Prelude Data.Maybe> maybe 7 id (Just 2) 2 

Example 2 : creating a function through a fold:

 Prelude Data.Maybe> :t foldr (.) id [(+2), (*7)] :: (Num a) => a -> a Prelude Data.Maybe> let f = foldr (.) id [(+2), (*7)] Prelude Data.Maybe> f 7 51 

We built a new function f , folding the list of functions together with (.) , Using id as the base case.

Example 3 : the basic case for functions as monoids (simplified).

 instance Monoid (a -> a) where mempty = id f `mappend` g = (f . g) 

Like our bend example, functions can be viewed as concatenated values, with id serving as an empty case and (.) Serving as an append.

Example 4 : a trivial hash function.

 Data.HashTable> h <- new (==) id :: IO (HashTable Data.Int.Int32 Int) Data.HashTable> insert h 7 2 Data.HashTable> Data.HashTable.lookup h 7 Just 2 

Hashtables require hashing. But what if your key is already hashed? Then pass the id function to fill as your hash method, with zero overhead.

Answer 2

If you manipulate numbers, in particular with addition and multiplication, you will notice the usefulness of 0 and 1. Similarly, if you manipulate lists, an empty list will be very convenient. Similarly, if you are manipulating functions (very common in functional programming), you will notice the same usefulness of id .

Answer 3

In functional languages, functions are first class values ​​that you can pass as a parameter. Therefore, one of the most common uses of id occurs when you pass a function as a parameter to another function to tell what to do. One of the options for what to do is most likely to be “just leave it alone” - in this case you pass id as a parameter.

Answer 4

Suppose you are looking for some kind of puzzle solution where you make a move at every step. You start with the position of candidate pos . At each stage, there is a list of possible transformations that you could do for pos (for example, a sliding piece in a puzzle). In a functional language, it is natural to represent transformations as functions, so now you can make a list of moves using a list of functions. If doing nothing is a legitimate step in this puzzle, then you will represent it with id . If you haven’t done this, you will have to treat “do nothing” as a special case that works differently from “do something”. Using id , you can handle all cases evenly in one list.

This is probably the reason that almost all uses of id exist. Handle "doing nothing" evenly with "something."

Answer 5

For another answer:

I often do this when linking multiple functions using composition:

 foo = id . bar . baz . etc 

over

 foo = bar . baz . etc 

This makes editing easy. Similar things can be done with other null elements, such as

 foo = return >>= bar >>= baz foos = [] ++ bars ++ bazs 

Answer 6

I can also help improve your golf score. Instead of using

($)

you can save one character with id.

eg.

zipWith id [(+1), succ] [2,3,4]

An interesting, more useful result.

Answer 7

Since we find good id applications. There is a palindrome here :)

 import Control.Applicative pal :: [a] -> [a] pal = (++) <$> id <*> reverse 

Answer 8

Imagine that you are a computer, i.e. you can follow the sequence of steps. Then, if I want you to remain in your current state, but I always have to give you instructions (I can’t just turn off the sound and let the time pass), what instructions do I give you? Id is a function created for this, to return the argument without changes (in the case of the previous computer, its state will be the argument) and for the name for it. This need arises only when you have high-order functions, when you work with functions without considering what is inside them, which makes you symbolically symbolize even the "do nothing" implementation. Similarly, 0, regarded as the quantity of something, is a symbol of the absence of quantity. In fact, in the algebra, both 0 and id are considered neutral elements of the operations + and ∘ (composition of functions), respectively, or more formally:

for all x type numbers:

• 0 + x = x
• x + 0 = x

for all f functions of type:

• id ∘ f = f
• f ∘ id = f

Answer 9

Whenever you need to have a function somewhere, but want to do more than just hold your spot (as an example, "undefined").

This is also useful as (soon) Dr. Stuart mentioned above when you need to pass a function as an argument to another function:

 join = (>>= id) 

or as a result of a function:

 let f = id in f 10 

(presumably you will edit this function later to make something more "interesting" ...;)

As others have noted, id is a great place holder when you need a function somewhere.