Haskell function returning Just pair of values if both arguments are Just, Nothing otherwise

Question

Define a function ``` pairMaybe :: Maybe a -> Maybe b -> Maybe (a,b) ``` that produces a `Just` result only if both arguments are `Just`, and a `Nothing` if either argument is `Nothing`. I’ve come up with: ``` pairMaybe (Just a) (Just b) = Just (a,b) pairMaybe (Just a) Nothing = Nothing pairMaybe Nothing (Just b) = Nothing ``` I’m not sure if this is the right way of writing it. Is there something wrong with this or is this the way to define this function? Also I think I’d probably like a better explanation of what this function can actually do, so if I called `pairMaybe` with two arguments, what arguments can they be? Of course they have to be of type `Maybe`, but what’s a good example?

Accepted Answer

Doing this via pattern matching is fine; you could simplify your code though by using

```
pairMaybe :: Maybe a -> Maybe b -> Maybe (a,b)
pairMaybe (Just a) (Just b) = Just (a,b)
pairMaybe _        _        = Nothing
```

That being said, your function actually just lifts the `(,)` function (which creates 2-tuples) into the `Maybe` monad, so you could also write

```
pairMaybe :: Maybe a -> Maybe b -> Maybe (a,b)
pairMaybe = liftM2 (,)
```

Haskell function returning Just pair of values if both arguments are Just, Nothing otherwise

Solution

Problem