Haskell: check if two lists are equal

I want to check if two lists A and B are equal, i.e., a1 == b1, a2 == b2,...

I have a working solution:

all (\x->x) zipWith $ (==) A B

Another idea is to do it recursively: a:as, b:bs ; check if a1==b1 and call the function with the remaining lists as and bs. But isn't there an easier and more readable way to do this?


You can just use == on them directly.

> [1, 2, 3] == [1, 2, 3]
> [1, 2, 3] == [1, 2]

This is because == is part of the Eq type class, and there is an Eq instance for lists which looks something like this:

instance Eq a => Eq [a]

This means that lists instantiate Eq as long as the element type also instantiates Eq, which is the case for all types defined in the standard Prelude except functions and IO actions.

First, hammar's answer is correct, so accept his answer please. (Edit: Which you have done, thank you.)

listA == listB

(I'm going to nitpick on small details in your question, mostly for the benefit of future beginners who find this page on Google.)

Second, A and B aren't lists: they start with upper case letters, so they cannot be variables. I'm going to call them listA and listB instead.

Third, there is a typo in your working solution: the $ should be before the zipWith, not after. The way it appears in your question results in a compile error. I think you meant this:

all (\x->x) $ zipWith (==) listA listB

Fourth, (\x->x) is better known as the function id.

all id $ zipWith (==) listA listB

Fifth, as Matvey points out, all id is the same as and.

and $ zipWith (==) listA listB

Sixth, these do different things when the lists have different lengths. Using (==) directly on lists will result in False, whereas zipWith will ignore the excess elements. That is:

[1,2,3] == [1,2]                   -- False
and $ zipWith (==) [1,2,3] [1,2]   -- True

Now, there are probably situations when you want the second behaviour. But you almost certainly want the first behaviour.

Finally, to emphasise, just use (==) directly on the lists:

listA == listB

Need Your Help

Inheritance - Handling child specific code in base class

design inheritance parent-child

We have a parent class - Platform. All the specific device class are inherited from this parent Class - Device1, Device2, ..... DeviceN.

How to get raw query string data and use it to build iframe source?

javascript jquery html iframe query-string

I have a scenario with a client where I'm trying to pass query string data from form 1 to form 2, to pre-fill the second form with contact info. Form 2 is iframed on the first form's "thank you" pa...

About UNIX Resources Network

Original, collect and organize Developers related documents, information and materials, contains jQuery, Html, CSS, MySQL, .NET, ASP.NET, SQL, objective-c, iPhone, Ruby on Rails, C, SQL Server, Ruby, Arrays, Regex, ASP.NET MVC, WPF, XML, Ajax, DataBase, and so on.