Stream: helpdesk (published)

Topic: ✔ typeof(aFunction)

Peter Goodall (Nov 16 2021 at 20:10):

if I check the type of an identifier which is a function:

julia> typeof(zero)
typeof(zero)

wouldn't generic function "zero" be more helpful?

Sebastian Pfitzner (Nov 16 2021 at 20:10):

but that's not the type

Peter Goodall (Nov 16 2021 at 20:18):

OK - but I feel short-changed by getting typeof(zero) echoed back. I don't think I got any more information...
if I do methods(zero) I seem to get the full description, but it's more than I need. What should I be asking?

Sukera (Nov 16 2021 at 20:20):

I guess that depends on what you want to know

Mason Protter (Nov 16 2021 at 20:20):

Basically, when you type

function foo(x)
    x + 1
end

that gets turned into something essentially equivalent to

struct var"typeof(foo)"
end
function (foo :: var"typeof(foo)")(x)
    x + 1
end
const foo = var"typeof(foo)"()

Mason Protter (Nov 16 2021 at 20:20):

all julia functions are just callable structs

Sukera (Nov 16 2021 at 20:20):

julia, unlike Haskell, doesn't have the arguments to a function as part of the type signature

Mason Protter (Nov 16 2021 at 20:20):

and when you define a closure, then the closed over data will end up as fields of that struct

Mason Protter (Nov 16 2021 at 20:21):

e.g.

julia> let x = 1
           f(y) = x + y
           dump(f)
       end
f (function of type var"#f#4"{Int64})
  x: Int64 1

Sukera (Nov 16 2021 at 20:21):

a "function" in julia is just the name - the individual things that have code in them are "methods", which are associated with a tuple of argument types

Mason Protter (Nov 16 2021 at 20:24):

this means that the above closure was equivalent to writing

struct var"f#4"{T}
    x::T
end
function (f :: var"f#4")(y)
    f.x + y
end
f = var"f#4"(x)

Peter Goodall (Nov 16 2021 at 20:27):

That's going to take me a while to digest :-) - very interesting...
So what do we call the informational string I get sent when I create a method/function - looks like this (can't remember exactly) generic function with 2 methods. Can I ask for that given the identifier?
I'm trying to avoid making unintended software jokes, like I did with zero.

Sukera (Nov 16 2021 at 20:28):

I'd guess a display string?

Sukera (Nov 16 2021 at 20:29):

it's really just an informational piece of text

Sukera (Nov 16 2021 at 20:29):

"generic" in this context means that you can add methods to it, if I'm not mistaken (and if I am, I'll be corrected probably)

Peter Goodall (Nov 16 2021 at 20:29):

@Sukera
help me solve the zero, one pun

Mason Protter (Nov 16 2021 at 20:29):

dump is a very useful function when you need to understand what something is

julia> dump(zero)
zero (function of type typeof(zero))

Sukera (Nov 16 2021 at 20:30):

also, ?zero

Sukera (Nov 16 2021 at 20:30):

in general, ? followed by any symbol in the REPL will display the associated documentation (or an autogenerated one, if none exists)

Peter Goodall (Nov 16 2021 at 20:30):

Great - thanks.
I really appreciate this generous coaching !!

Notification Bot (Nov 16 2021 at 20:31):

Peter Goodall has marked this topic as resolved.

Peter Goodall (Nov 16 2021 at 20:41):

@Mason Protter Now I get what you were showing me with closure/struct

Last updated: Oct 02 2023 at 04:34 UTC