Passing two arguments of differing types

Question

I want to create a function, let's call it altHead, that takes 2 variables and outputs a variable such that:

a -> [a] -> a

I want to call it like this:

altHead 3 [1,2,3,4]

What it should do is to look at the first variable, and the list. If the list is empty, then return the first var's value; otherwise, return the list.

In this case, I've written the following code.

altHead :: a -> [a] -> a
altHead (x:xs) = if null[xs] then [x] else [xs]

However, I believe x:xs only looks through the list.

I'm a bit stuck as to how to change this so that it looks at the first value as well, and how to include the first value in the function.

Hint: don't use if/then/else at all. Pattern matching is all you need. — Carl
– Carl, Commented May 13, 2017 at 22:34
(x:xs) doesn't bind two arguments, it binds the head and tail of a single argument that must be a list. — Wyzard
– Wyzard, Commented May 13, 2017 at 22:40
@Wyzard Aye, this is the conclusion I came to as well. Just wasn't sure how to include the first var in there too... — R.Sama
– R.Sama, Commented May 13, 2017 at 22:44

zbw · Accepted Answer · 2017-05-13 22:47:50Z

3

You want something like:

altHead :: a -> [a] -> a
altHead a xs = if null xs then a else head xs

Or alternatively, using pattern matching on the list:

altHead :: a -> [a] -> a
altHead a [] = a
altHead a (x:xs) = x

answered May 13, 2017 at 22:47

zbw

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7 Comments

Wyzard Over a year ago

The last line of the second example can also be written altHead _ (x:_) = x, since neither a nor xs are actually used.

zbw Over a year ago

Yes, I just thought it helped readability/understanding to leave them as variable names. (It also shows the (x:xs) pattern that the OP used.)

R.Sama Over a year ago

I think I now understand something I had fundamentally MISunderstood previously. Thanks for this, saw it just as I had finished my version, and it was a little bit off, but otherwise ok. Quick Q: I realise this isn't how typing would work normally, but is it possible for it to return the entire LIST if it's not empty, but otherwise only return x?

Wyzard Over a year ago

You can't have a function that sometimes returns a list and sometimes returns a single value, because those are different types. But you could change the signature to a -> [a] -> [a] and return [a] when the list is empty.

R.Sama Over a year ago

Thank you, this is what I needed. Gonna learn me some more Haskell, cheers guys!

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