Haskell tail function for empty lists

Question

I have a problem with a function that should only return the tail of a list. The functions is myTail and should give a useable result, even if the input is an empty list.

I want to understand all 3 ways: pattern matching, guarded equation and conditional expressions

this works:

> myTail_pat :: [a] -> [a]

> myTail_pat (x:xs) = xs
> myTail_pat [] = []

But this:

> myTail_guard (x:xs)   | null xs = []
>               | otherwise = xs

gives me the error: Program error: pattern match failure: myTail_guard [] How can i declare the function without patterns?

Thank you.

Might help you: http://cs.anu.edu.au/Student/comp1100/haskell/tourofsyntax.html#Guards — Artelius, Nov 18 '09 at 22:28

score 17 · Accepted Answer · answered Nov 18 '09 at 22:17

17

The pattern x:xs does not match the empty list. You'd need to do:

myTail_guard xs
  | null xs   = []
  | otherwise = tail xs

answered Nov 18 '09 at 22:17

Chuck

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If you want to avoid `tail xs` I think you could replace it with `xss where (_, xss) = xs` – Artelius Nov 18 '09 at 22:20
3

@Artelius: that should be `xss where (_:xss) = xs`. – porges Nov 18 '09 at 23:13
or lazy patterns – PyRulez Dec 16 '17 at 02:57
What reason would someone what to use `xss where (_:xss) = xs` over `tail xs` for this purpose? – Lightbulb1 Apr 09 '18 at 20:43

score 13 · Answer 2 · answered May 21 '17 at 23:57

13

drop 1 is safe

drop 1 []
-- result: []

answered May 21 '17 at 23:57

Ilya Kharlamov

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Is there any negative effects to using this approach vs the normal tail function or pattern match? – Lightbulb1 Apr 09 '18 at 20:41
@Lightbulb1 please define what is a "negative effect", is that a side effect or what. The only difference between "tail" and "drop 1" is that tail throws an error on an empty list. – Ilya Kharlamov Apr 09 '18 at 21:08
Thanks for your response. I was thinking in terms of performance. – Lightbulb1 Apr 09 '18 at 22:06
1

Although I believe it's implemented using a linked list so the performance is the same, you must not have the implementation performance under consideration when writing a program in Haskell. In a more philosophic way, in an imperative language you tell the compiler "how to do it" and in a functional one you tell "what you want to get done". If performance is your priority, then rather take ASM or C golang or something similar. – Ilya Kharlamov Apr 10 '18 at 21:40

Haskell tail function for empty lists

2 Answers2