What If We Could Mark Methods As Pure?

gistfile1.md

What If We Could Mark Pure Methods?

Me

• Craig Buchek
• @CraigBuchek
• github.com/booch

What is a pure method?

• No side-effects
• Does not mutate state
• Idempotent - can run multiple times, always getting same result

def method1
  1 + 1
end

def add1(x)
  x + 1
end

def not_pure1
  Random.rand
end

def not_pure2(x)
  self.x = x
end

def not_pure3
  puts "Hello!"
end

Why?

• Could automatically memoize pure methods
• Could be run in parallel, theoretically
• Functional Programming

Marking pure methods

def method1(options)
  # ...
end
pure :method1

# Ruby 2.1+
pure def method2(options)
  # ...
end

@pure
def method2(self, options):
  # ...

But

• C++ allows marking pure methods
• C++ has const everywhere

Marking impure methods

• We'd be encouraged to default to pure methods

Inference

• Could we infer purity via static analysis?

But

• We'd really have to do it in the interpreter
• Fairly dramatic change to the language semantics
• Considering trying to add it to Rubinius or Crystal

Going even further

• Marking classes as mutable/immutable
  • More value objects!
• Marking instances as mutable/immutable:

x = User.new('Craig')   # immutable
y = User.new!('Craig')  # mutable