5

I'm interested in becoming more familiar with functional programming as a paradigm, so I'm trying to introduce a more functional style to some of my projects. I'm struggling to understand how to handle side effects with a database.

I have some functions that kind of look like this:

                 db query
                     +
db query             |
   +                 |
   |                 v
   |      +--------->a()
   v      |
f(type)+--+
          |
          +--------->b()

The trouble is that both f and a are non-pure functions because they need to do database queries. I've seen some functional projects that work by having all the state in a single place and the rest of the application takes bits and pieces of state as function parameters. I can replicate something like that here by putting all the queries in f for example, but since b doesn't need the database queries used by a, this would be really inefficient.

Is there a pattern for handling database access in functional programs?

20
  • 1
    How inefficient it can be if you load all required data and pass it to f as a parameter? In most of the cases it is not significant, especially in cases where you can load data asynchronously.
    – Fabio
    Feb 6 at 19:14
  • 1
    @GregBurghardt, I think by "side effects" OP mean all kinds of access to external resources(database) which can return "unpredictable" result.
    – Fabio
    Feb 6 at 20:35
  • 1
    @Steve, sorry i would say about "pure function' as function which for same input always return same output. SQL SELECT statement returns data from the file system, which can be updated between select statements.
    – Fabio
    Feb 6 at 22:03
  • 1
    @Steve, Today SELECT COUNT(*) FROM MyTable returns 99. Next week, same query(with same arguments as you said) SELECT COUNT(*) FROM MyTable will return 120. With pure function sum(12, 30) returns 42 today and next week it will also return 42. With pure function I can save final result and never call this function anymore for arguments 12 and 30, but I can not do it for SQL statements, I need to execute actual sql statement again.
    – Fabio
    Feb 7 at 21:28
  • 1
    @Steve, internally yes, but may be not from the database consumer perspective, who sends "SELECT COUNT(*) FROM MyTable" string to the database and receives different results at different time, which is the case for OP's example.
    – Fabio
    Feb 7 at 22:31
7

Extract important business logic out of application dependencies and "wrap" it with database calls or calls to other external resources.

-- Load data
-- Process data
-- Save data based on processed result

Code structure will look like below:

# --> logic with side effects
data = load_data_from_external_resources()

# --> pure logic without side effects
result = business_logic.process(data) 

# --> logic with side effects based on result
if result.has_records
  save(result.records)
end

if result.should_notify_others
  notify_others(result.message)
end

With such approach business logic will be totally independent of application technologies(database, file system or third parties) in design and runtime.

For very complicated cases where loading all required data is not possible because of performance issues we can introduced small bits of business logic and "stack" layers of application and business logic on each other

# --> load minimum required data
data = load_minimum_data()

# --> pure logic without side effects
next_steps = business_logic.determine_next_steps(data) 

if next_steps.need_more_data?
  # --> load more data
  data = data + load_more_data()
end

# --> pure logic without side effects
result = next_steps.process(data) 

# --> logic with side effects based on result
if result.has_records
  save(result.records)
end
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  • 2
    Downvoter, please explain what is wrong with the answer, I will be happy to fix or delete it.
    – Fabio
    Feb 7 at 17:50
  • 2
    I don't see what's wrong with it. Have an upvote from me. Feb 7 at 19:05
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    This kind of approach is often called “functional core, imperative shell”. Such an architecture is a great fit for many problems, and corresponds nicely to concepts such as hexagonal architecture or onion architecture.
    – amon
    Feb 7 at 19:49
  • Have an upvote from me too, and damn the silent downvoters!
    – Steve
    Feb 8 at 1:02
6

f only needs to do a database query because it needs a which needs a database query.

a only needs to do a database query because no one has passed it what the result of that query would be.

Solution: pass a what it needs.

Side effects exist in every useful program. Otherwise you'd see nothing on the screen. The issue is where they exist. Programming functionally isn't eliminating all side effects. It's being formal with them. Only permitting them to exist in well controlled places away from all your interesting, needs testing and debugging, business logic. That's where you keep your pure functions. Move side effects to boring structural code that people understand at a glance.

The biggest impact here is that a no longer controls when the database is queried. But if you want a to be pure that's the cost it comes at.

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