A beginners guide to API over-engineering

March 9, 2016
Yuras Shumovich

It is hard to quantify over-engineering. But it is crucial for library API designer to identify over-engineering as early as possible, and keep API simple and easy to use.

Over-engineering is about unnecessary complexity. In theory it is easy to avoid: each time you see multiple alternatives and you are not sure which one is better, you should select the simplest one. It practice we don’t always can say what alternative is simpler. Often we even don’t see other alternative, so the decision is made without any complexity analyze.

It this post I’ll try to do a bit unusual thing. I’ll take a problem, and make the simples API design I can imaging. Then I’ll make a series of redesigns, successively increasing complexity, and we’ll discuss whether the complexity was necessary. You can decide yourself at which point the API becomes “over-engineered”.

The problem is question is a key/val database. The API we’ll start looks like the next:

data DB
open :: FilePath -> IO DB
close :: DB -> IO ()
set :: DB -> ByteString -> ByteString -> IO ()
get :: DB -> ByteString -> IO (Maybe ByteString)

It is so simple, that I’m even not going to describe what it is doing. One even don’t need to know anything about monads to use it (I assume that do-notation doesn’t count). Now lets start iterating on the API.


The first thing we notice is that three functions accept DB as an argument. What is user uses this functions in a row, like this:

withDB :: FilePath -> (DB -> IO a) -> IO a
withDB path use = bracket (new path) close use

withDB "some.db" $ \db -> do
  set db "key1" "val1"
  set db "key2" "val2"
  set db "keyN" "valN"

It is a boilerplate! We can avoid it using Reader monad transformer:

set :: ByteString -> ByteString -> Reader DB IO ()
get :: ByteString -> Reader DB IO (Maybe ByteString)

Now user doesn’t have to pass DB explicitly:

withDB :: FilePath -> Reader DB IO a -> IO a
withDB path use = bracket (new path) close (runReader use)

withDB "some.db" $ do
  set "key1" "val1"
  set "key2" "val2"
  set "keyN" "valN"

Kind of cool. But also we increased complexity of the API. Now user has to know about monads, transformers, lifting, etc. Even worse, we introduce new abstraction, implicit environment, which is inadequate to out problem. Imaging that you want two databases at the same time. With the initial version of API it is trivial:

withDB "some1.db" $ \db1 -> do
  withDB "some2.db" $ \db2 -> do
    maybe_v1 <- get db1 "key1"
    set db2 "key1" (fromMaybe "" maybe_v1)

    maybe_v2 <- get db1 "key2"
    set db2 "key2" (fromMaybe "" maybe_v2)

With the new API we’ll need a bit of code golfing to do something like that. Does a bit of boilerplate worse the complexity? How will users use the API more often? As a library writers, we can’t know, so it is better to stick with simpler version, the initial one. A bit of boilerplate is far cheaper then wrong abstraction. But lets accept this additional complexity, and make the next step.


At the previous step we introduced another issue. What if user uses his own transformer stack? We can’t compose different transformer stacks, we can only nest them. Lets introduce mtl!

set :: (MonadReader DB m, MonadIO m) => ByteString -> ByteString -> m ()
get :: (MonadReader DB m, MonadIO m) => ByteString -> m (Maybe ByteString)

Now API user has to understand type classes and mtl. Type signature becomes longer. Seasoned haskellers will certainly not find it problematic, but they are able to solve the issue with transformer stacks! By contrast newcomers will probably not be able to use the API at all. Note that the original design we started with doesn’t suffer from the issue with transformer stacks composition, simply because it doesn’t use transformers. Is the complexity necessary here? I think it is not, but you can decide for yourself.

Structured data

It is rare to store plain strings in database. Usually we store some structured data, so we need a way to serialize and deserialize it. The standard way to handle serialization is to use type classes:

set :: (MonadReader DB m, MonadIO m, Serialize v) => ByteString -> v -> m ()
get :: (MonadReader DB m, MonadIO m, Serialize v) => ByteString -> m (Maybe v)

Looks good. Here I used Serialize type class from cereal package. Wait, but what if user uses binary package? What about the next:

set :: (MonadReader DB m, MonadIO m, Binary a) => ByteString -> a -> m ()
get :: (MonadReader DB m, MonadIO m, Binary a) => ByteString -> m (Maybe a)

Which one to use? Should we provide both variants? It will be really bad idea, because it will introduce incidental dependencies on cereal and/or binary. Should we introduce our own type class? Does it worse the complexity? No, it doesn’t. Serialization is not the core functionality for our library, so our users should not be forced to learn one more serialization API just to get a value from the database.

(A hint: key name sometimes is a structured data too, what about encoding and decoding it too just like we do for values?)


Suppose we want to count all operations we do on the database. It could be useful for example for performance monitoring. We can just count everything inside MVar and provide an API to access it:

getPerformanceCounters :: DB -> IO Counters

But we should keep effects under control! Lets use free monad to solve the issue:

data Op next
  = Set ByteString ByteString next
  | Get ByteString (Maybe ByteString -> next)
  deriving (Functor)

interpret :: Free Op a -> IO a

Ops, we just moved the core functionality of our library out to interpreter. Now the library does everything except writing to and reading from database. I hope it is obviously over-engineering, so I’ll stop right here.


So monad transformers, free monads, etc. are bad and should be avoided? No, they are cool and useful. But as any other tool, they have their own application areas. There is nothing wrong in tranformer stack, mtl or free monad used in implementation, but please think twice before exposing them to external API. And certainly there are legitimate use cases where you really need them in API. Design API thoughtfully and keep it simple.

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