PrePostCall

PrePostCall is a package which offers an intuitive syntax for making preceding and subsequent function calls to other functions using macros.

Here is a very simple example showing how to define a macro with PrePostCall which checks that

• all passed arguments to a function are positive and
• the return value of a function is not Inf

Pre and post calls can be defined with @pre and @post respectively. So first the new macros are defined:

@pre positive(x::Number) = x<0 && error("Passed values must be positive!")
@post notInf(x::Number) = isinf(x) && error("The return value is Inf")

Now both macros @positive and @notInf can be applied to another function:

@notInf @positive x y z function foo(x,y,z)
    (x+y)/z
end

Calls to foo with various arguments now result in the following:

julia> foo(1,2,3)
1.0

julia> foo(1,-2,3)
ERROR: Passed values must be positive!

julia> foo(1,2,0)
ERROR: The return value is Inf

In the following toy example you have a mutable struct where one field can either be an Int or nothing. A function that is called with this mutable struct should only be usable if the field is not nothing. Another function that is called with this mutable struct should only be usable if the field is not nothing and at least has a value of 3. (This example is minimized to illustrated the usage of PrePostCall.)

First define the struct:

mutable struct Bar
    val::Union{Int,Nothing}
end

Then define the check functions:

@pre alive(b::Bar) = b.val == nothing && error("Bar must not be nothing")
@pre large(b::Bar) = b.val < 3 && error("The value of bar must be >= 3")

The actual functions used on the mutable type can now be created with a clear, dense and easily readable definition:

@alive addOne(b::Bar) = b.val += 1
@alive @large addTen(b::Bar) = b.val += 10

If no variable names are given for the newly created macros, the variables checked are assumed to have the same name as the ones used on the @pre (or @post) definitions.

Calls to the defined function with various Bar-types now result in the following:

julia> a = Bar(1)
Bar(1)

julia> addOne(a)
2

julia> a.val = nothing

julia> addOne(a)
ERROR: Bar must not be nothing

julia> b = Bar(1)
Bar(1)

julia> addTen(b)
ERROR: The value of bar must be >= 3

julia> addOne(b)
2

julia> addOne(b)
3

julia> addTen(b)
13

julia> b.val = nothing

julia> addTen(b)
ERROR: Bar must not be nothing