In my (generally functional) code, I frequently find myself asking for "An
object like X but with Y different", eg. "that image but with the
background set to transparent" or "that dataframe but with all inactive
customers' profit set to 0". The usual idiom is:
img2 = copy(img)
img2[img2.==black] = transparent
... some_function(img2)
I've written a macro that lets me write instead
some_function(@assign(img[img.==black], transparent))
and it expands into the code above. This is very convenient, but it hits a
snag with immutable types, as the assignment fails. eg. @assign(obj.field,
5) doesn't work. Thanks to Julia introspection, I can programmatically get
the fields, change the one that is @assigned to, and pass them to the
constructor, but that only works with the default constructor. This will
fail
immutable Foo
a
b
Foo(x) = new(x,x)
end
f = Foo(1)
@assign(f.b, 3)
(arguably this @assign in particular violates Foo's invariant, but that's
not the point)
Is there any way to do this differently? In particular, can I access the
"default constructor" for a type somehow?
Cédric
