Tobias Dammers


Object-Oriented Haskell

Part 1: Objects, Fields, Methods, Interfaces

Oct 17, 2017

Object-oriented Haskell, you say? But isn’t Haskell a functional programming language? Aren’t functional programming and object-oriented programming mutually exclusive?

Well; no, they are not, and I will show you how it is possible to write object-oriented code in Haskell using only a minimal set of utility code, and without giving up much of Haskell’s pure functional benefits.

What Is OOP, Anyway

When people talk about OOP (Object-Oriented Programming), you will rarely see the term actually defined, and it seems that there isn’t a lot of agreement on what it really means.

Well then, for the purpose of this article, we will need a working definition; rather than going all crazy on this, I will write down a few key features for the particular flavor of object-oriented programming that I consider crucial, and then we will see how we can implement that in Haskell.

Case Study: SQL Query Generation DSL

For our case study, we will implement a little EDSL for generating SQL queries, such that user code can provide queries in a backend-agnostic form, and our code will render them as backend-specific SQL query strings. For illustration purposes, we will only support simple SELECT queries; the data types to model these look like this:

Great, this should be enough to write single-table SELECT statement with some basic support for WHERE clauses, LIMIT, and ORDER BY. For example:

Which should render to SQL similar to this:

Now we want a function renderSqlQuery :: SqlDialect -> SelectQuery -> String, such that the SqlDialect determines the details of how the query gets rendered.

First step: Defining An Interface.

So far, everything we’ve done is uncontroversial plain old Haskell. Now we need to define an interface for SqlDialect, and a good way to model this is using a plain old data type:

data SqlDialect
  = SqlDialect
      { renderSqlQuery :: SqlDialect -> SelectQuery -> String

We will extend this type later; note, for now that the renderSqlQuery field is a function that takes an additional argument of type SqlDialect. This is on purpose, as we will see later. For now, it means that if we want to call this function, we will need to pass the SqlDialect object twice:

This is a bit awkward, and we will solve this in a minute. But first, we need to address one other thing.

Our interface so far is not polymorphic, all we can do is provide an SqlDialect value and do things with it, but it’s still a dumb old Haskell data type. To make things polymorphic yet type-safe, we will need some more juice; particularly, we will define a typeclass Is, which tells the compiler that a certain type implements a certain interface:

Now whenever an instance foo `Is` SqlDialect exists, we can write:

And now we can resolve the awkwardness by writing a little function that puts these things together:

Or, for convenience, as a binary operator:

Now we can write:

…which, in something like Java, might look something like:

For completeness sake, let’s define:

That is, every interface implements itself.

Implementing The Interface (Part 1)

We’ll start with a naive “vanilla” SQL dialect that has no state of its own.

Wow, that was easy! Well, we haven’t implemented anything yet, so let’s:

vanillaRenderSelect :: SelectQuery -> String
vanillaRenderSelect query =
  "SELECT " ++
    intercalate ", " (map vanillaQuoteColumn $ selectColumns query) ++
    " FROM " ++
    vanillaQuoteTable (selectTable query) ++
    vanillaRenderWhere (selectWhere query) ++
    vanillaRenderOrders (selectOrder query) ++
    vanillaRenderLimit (selectLimit query)

vanillaRenderWhere :: Condition -> String
vanillaRenderWhere Always = ""
vanillaRenderWhere cond = "WHERE " ++ vanillaRenderWhereCond cond

vanillaRenderWhereCond Always =
vanillaRenderWhereCond (Equals a b) =
  vanillaRenderValue a ++ " = " ++ vanillaRenderValue b
vanillaRenderWhereCond (Not cond) =
  "NOT (" ++ vanillaRenderWhereCond cond ++ ")"
vanillaRenderWhereCond (IsNull a) =
  vanillaRenderValue a ++ " IS NULL "
vanillaRenderWhereCond (And a b) =
  "(" ++ vanillaRenderWhereCond a ++ " AND " ++ vanillaRenderWhereCond b ++ ")"
vanillaRenderWhereCond (Or a b) =
  "(" ++ vanillaRenderWhereCond a ++ " OR " ++ vanillaRenderWhereCond b ++ ")"

vanillaRenderOrders [] =
vanillaRenderOrders orders =
  "ORDER BY " ++ intercalate ", " (map vanillaRenderOrder orders)

vanillaRenderOrder (Order field Asc) = vanillaQuoteColumn field
vanillaRenderOrder (Order field Desc) = vanillaQuoteColumn field ++ " DESC"

vanillaRenderLimit Nothing = ""
vanillaRenderLimit (Just (Limit n)) = "LIMIT " ++ show n
vanillaRenderLimit (Just (LimitOfs ofs n)) = "LIMIT " ++ show ofs ++ " " ++ show n

vanillaRenderValue (ColumnRef field) = vanillaQuoteColumn field
vanillaRenderValue (Param param) = vanillaQuoteParam param
vanillaRenderValue (Literal val) = vanillaQuoteLiteral val

vanillaQuoteColumn col = "\"" ++ col ++ "\""
vanillaQuoteParam = const "?"

vanillaQuoteTable table = "\"" ++ table ++ "\""

-- Not actually accurate, we would also have to perform escaping, but
-- for demonstration purposes this will have to do.
vanillaQuoteLiteral val = "'" ++ val ++ "'"

Cool. Now we could write our instance VanillaSql `Is` SqlDialect, but before we do, let’s take a step back. At some point, we will want to write other SQL dialect implementations, but they will share a lot of code with the vanilla flavor - for example, we could probably reuse most of the above for MySQL, but we would want to override the quoting behavior such that it follows the MySQL custom of using backticks for table and column names. The way we’ve written our SQL generation functions, this isn’t possible, because the vanilla renderer functions always call into other vanilla functions - we need those nested calls to somehow be aware of the runtime object they are being called on. In other words, we need runtime dispatch and open recursion, and this is why we added the additional argument to our methods in the interface definition.

Virtual Methods

In order to make our methods virtual, that is, making the choice of implementation dependent on the runtime object, we need to pass an additional copy of the object around, which we will use for open-recursive calls to other methods. By convention, we will name this additional argument this.

So, let us first extend the interface.

Note that every method takes the additional this argument.

Implementing The Interface (Part 2)

Armed with this, we can extend our Vanilla SQL implementation to match the type signatures:

vanillaRenderSelect :: SqlDialect -> SelectQuery -> String

vanillaRenderSelect this query =
  "SELECT " ++
    intercalate ", " (map (this ==> quoteColumn) $ selectColumns query) ++
    " FROM " ++
    (this ==> quoteTable) (selectTable query) ++
    (this ==> renderWhere) (selectWhere query) ++
    (this ==> renderOrders) (selectOrder query) ++
    (this ==> renderLimit) (selectLimit query)

vanillaRenderWhere :: SqlDialect -> Condition -> String
vanillaRenderWhere this Always = ""
vanillaRenderWhere this cond = "WHERE " ++ (this ==> renderWhereCond) cond

vanillaRenderWhereCond this Always =
vanillaRenderWhereCond this (Equals a b) =
  (this ==> renderValue) a ++ " = " ++ (this ==> renderValue) b
vanillaRenderWhereCond this (Not cond) =
  "NOT (" ++ (this ==> renderWhereCond) cond ++ ")"
vanillaRenderWhereCond this (IsNull a) =
  (this ==> renderValue) a ++ " IS NULL "
vanillaRenderWhereCond this (And a b) =
  "(" ++ (this ==> renderWhereCond) a ++ " AND " ++ (this ==> renderWhereCond) b ++ ")"
vanillaRenderWhereCond this (Or a b) =
  "(" ++ (this ==> renderWhereCond) a ++ " OR " ++ (this ==> renderWhereCond) b ++ ")"

vanillaRenderOrders this [] =
vanillaRenderOrders this orders =
  "ORDER BY " ++ intercalate ", " (map (this ==> renderOrders) orders)

vanillaRenderOrder this (Order field Asc) = (this ==> quoteColumn) field
vanillaRenderOrder this (Order field Desc) = (this ==> quoteColumn) field ++ " DESC"

vanillaRenderLimit this Nothing = ""
vanillaRenderLimit this (Just (Limit n)) = "LIMIT " ++ show n
vanillaRenderLimit this (Just (LimitOfs ofs n)) = "LIMIT " ++ show ofs ++ " " ++ show n

vanillaRenderValue this (ColumnRef field) = (this ==> quoteColumn) field
vanillaRenderValue this (Param param) = (this ==> quoteParam) param
vanillaRenderValue this (Literal val) = (this ==> quoteLiteral) val

vanillaQuoteColumn this col = "\"" ++ col ++ "\""
vanillaQuoteParam this = const "?"

vanillaQuoteTable this table = "\"" ++ table ++ "\""

-- Not actually accurate, we would also have to perform escaping, but
-- for demonstration purposes this will have to do.
vanillaQuoteLiteral this val = "'" ++ val ++ "'"

And now, writing our instance is easy:


Now things get interesting: with the way we have separated query generation out, we can now build other SQL dialect implementations on top of our basic “vanilla” flavor, overriding methods selectively:

Now the SqlDialect instance for MySQL inherits everything from VanillaSql, except the quoting rules for columns and tables. And because we’re explicitly passing this around, the calls to quoteColumn and quoteTable are resolved based on that inside all the other calls, so the code does the right thing, just like you’d expect.


We’re certainly not done at this point, but so far, we have:

We have also managed to avoid a few problems commonly associated with OOP:

And we have retained most of the advantages of Haskell:

There are also some loose ends that we haven’t addressed yet:

I will go into these questions in future posts.

Oh, and by the way, the framework laid out in this blog series is also available on Hackage, under the name boop.

Revision History

  • Oct 17, 2017
  • Oct 18, 2017: Cosmetic improvements
  • Oct 19, 2017: Added conclusion
  • Oct 26, 2017: Subtitle