How do you match against the type "Either a b"?

Question

The purpose for this particular portion of code is to make the `size` function more efficient than simply counting all the elements in `elems`. I've settled on summing the two types that make up the list, but I can't seem to create the signature of the size function. ``` instance (Finite a, Finite b) => Finite (Either a b) where elems = combineLists [Left x | x <- elems] [Right x | x <-elems] size ??? = (size a) + (size b) ``` From Prelude, we know that `Either a b = Left a | Right b`. The first thing I tried was to match `Either`, but of course it is a type, so that doesn't work. Next, I tried `((Left a) | (Right b))`, but no go on that either. Nothing else seems to match the type `Either a b`. I was able to get `size (Left a)` to compile, but since it's missing the `b` component, I receive the error: ``` Ambiguous type variable `b' in the constraint: `Finite b' arising from a use of `size' at :1:0-12 ``` which of course makes sense in the context, but I really have no c…

Accepted Answer

Something of type `Either a b` is either a `Left a` or a `Right b`, so you have two cases that can be handled separately:

```
size (Left x) = size x
size (Right x) = size x
```

The error about the ambiguous type variable is a separate issue. If you just type something like `size (Left 1)` into the interpreter, the system can't deduce what the "right" type of that `Left 1` value would be. It could be `Either Int anything` and as long as it is not known what type that `anything` is, it cannot be checked if it is in class `Finite` (which is required by `size`).

You can avoid that problem by specifying an explicit type signature:

```
size (Left 1 :: Either Int String)
```

How do you match against the type "Either a b"?

Solution

Problem