InControl

Intro

This package lets you customize the behavior of control statements like if and while by dispatching to user-defined functions based on the type of the predicate.

It fixes a hole in the generic programming toolkit of Julia: while multiple-dispatch is a great tool at letting you customize how a generic function will behave when passed your custom type as an argument, it fails if the function uses the argument as the predicate in an if or while statement as those statements require a Bool in the predicate slot.

Usage

Decorate the function with control structures you want to override with @control. Then:

• Easy case: You want the normal behavior of control statements, except you want the predicate to be convertible to a boolean rather than literally being a boolean. In that case, define a convert(::Type{Predicate}, x) method:

Base.convert(::Type{Predicate}, x::Nullable{Bool}) = get(x)
@control function f(x)
  if x
    1
  else
    -1
  end
end

@assert f(Nullable(false)) == -1

• General case: You want to totally customize the semantics of control statements, including possibly turning off the short-circuit behavior. Define a custom method handle_if(predicate::CustomPredicateType, consequent, alternative):

immutable IfTrace
  predicate
  consequent
  alternative
end

immutable TracedValue{T}
  value::T
end  

function InControl.handle_if(predicate::TracedValue, consequent, alternative)
  IfTrace(predicate.value, consequent, alternative)
end

>> f(TracedValue(:my_test))
IfTrace(:my_test, 1, -1)

while loops are also customizable via handle_while(predicate, loop_body).

Examples

See runtests.jl for additional examples.

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By Jon Malmaud (@malmaud)