Mock Database connections in haskell

Question

I am trying to write some simple code in haskell where there is a function performs a simple database query. In order to unit test Iam using HUnit but not sure how I can mock the database connection and query response.

Answer 1

Pass the function that performs the database query as a parameter to your code, instead of having it "hardwired". During tests, pass a mock function to the code. In production, pass a function that really performs the database query.

This new function parameter should be at the right level of abstraction. For example, something like

queryDbForClient :: Connection -> SQLString -> ClientId -> IO (Maybe RowData)

is possibly a bad idea, because it still forces the code under test to be aware that things like Connections and SQL strings exist. Something like this

findClientById :: ClientId -> IO (Maybe Client) 

would likely be better, because then you are free to pass functions that don't use db-specific Connection types at all. They could, for example, be backed by an in-memory reference.

Notice that you can build findClientById out of queryDbForClient by partial application and mapping the result a little. But this should be the task of some setup code, not of the main code that you want to test.

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Once you start passing functions in this way for the sake of testability and configurability, some common issues start to appear. For example, what if I have multiple dependencies? Passing them all as positional parameters is a chore. This might lead to passing them together in a record-of-functions, perhaps using Has-like typeclasses so as not to tie your main code to any particular record.