I have defined following class in Haskell:

```
class SearchList searchEntity where
searchList :: [ArtifactTree] -> searchEntity -> [Artifact]
```

I have two equal implementations of this class for different entities:

```
instance SearchList Version where
searchList [] _ = []
searchList (artifactTree:[]) element = searchElement artifactTree element
searchList (x:xs) version = (searchElement x version) ++ (searchList xs version)
instance SearchList Artifact where
searchList [] _ = []
searchList (artifactTree:[]) element = searchElement artifactTree element
searchList (x:xs) artifact = (searchElement x artifact) ++ (searchList xs artifact)
```

Is there a way to combine two equal instances into one to avoid code duplication? For example, I am looking for something similar to

```
instance SearchList {Version, Artifact} where
searchList [] _ = []
searchList (artifactTree:[]) element = searchElement artifactTree element
searchList (x:xs) version = (searchElement x version) ++ (searchList xs version)
```

Answer:

As Reid Henrichs said, there is no way to do this in general, and it's probably not necessary in this case. However, there are a couple things that can be very helpful.

Don't get scared off by the crazy name. The `Maybe`

type, for example, is "higher-kinded", because you can't have a value of type `Maybe`

—it needs to have type `Maybe a`

for some `a`

. There are two ways to deal with type classes for higher-kinded types. One way is to actually make a higher-kinded class. Another is to make a polymorphic instance. So for example, you can have

```
class Foo f where
foom :: f a -> Int
instance Foo [] where
foom lst = length lst
```

or you can have

```
class Bar t where
boom :: t -> Int
instance Bar [a] where
boom lst = length lst
```

Suppose you have a class, and want the instances for lists of `Int`

and lists of `Bool`

and lists of `[Bool]`

to all act in about the same way, but other sorts of lists to do something else. You can write list functions implementing that common behavior, and then write three tiny little instance declarations that use the functions.

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