Haskell清单问题

Question

I'm new to Haskell and I'm trying to write a simple function that takes a list of integers and returns a list such that the first element is added to all the elements of the list.

This is what I have so far.

addFirstEl [] = []
addFirstEl (x:xs) = [x + x | x <- xs]

Unfortunately all this has succeeded in doing is returning a list without the first element and the other elements being doubled.

Answer 1

The binding of x in the list comprehension is hiding the variable x from the pattern. Try this instead:

addFirstEl [] = []
addFirstEl (x1:xs) = [x1 + x2 | x2 <- xs]

Edit

In response to you comment

the first element still gets removed from the returned list

In (x1:xs) , xs is the remainder or tail of the list. It is all the elements after x1 , which is the head . If you want to add x1 to all the elements including itself, you could write

addFirstEl [] = []
addFirstEl (x1:xs) = [x1 + x2 | x2 <- (x1:xs)]

or in terms of head

addFirstEl [] = []
addFirstEl xs = [head xs + x | x <- xs]

Answer 2

Try this:

addFirstEl [] = []
addFirstEl l@(x:_) = [x + x1 | x1 <- l ]

or

addFirstEl [] = []
addFirstEl l@(x:_) = map (+ x) l