we should have better documentation and error message for with-decorator

[pyx]

Scenario 1

hy 0.14.0 using CPython(default) 3.6.5 on Linux
=> (with-decorator identity (defn a [] 42))
=> (with-decorator identity (fn [] 42))
  File "<input>", line 1, column 1

  (with-decorator identity (fn [] 42))
  ^----------------------------------^
HyTypeError: Decorated a non-function

It works on anonymous functions before (for example, 0.12.1), I believe decorating (fn ...) is useful sometimes. In python, there is no 'auto return', no multi-line lambda and def is not an expression, so we must use a named function, and the name is usually of no importance beside being used to be returned. To me, it looks good enough to have an fn as the last form.

For Example:

(defn my-decorator [f]
  (with-decorator (functools.wraps f)
    (fn [&rest args &kwargs kwargs]
      ...)))

I think we should support fn (as before) here, if not, we should mention that in the documentation, and using a better error message, fn is obviously a function.

Scenario 2

=> (with-decorator identity (defclass C))
=> (defclass C)
=> (with-decorator identity C)
  File "<input>", line 1, column 1

  (with-decorator identity C)
  ^-------------------------^
HyTypeError: Decorated a non-function

Here, feeding a type that's already been defined (not on the spot) raises an exception, which is reasonable BTW, but we should have more descriptive error message, with-decorator supports classes, too, and the wording implies only functions can be used here.

[Kodiologist]

I believe decorating (fn ...) is useful sometimes.

It shouldn't be, or else something is wrong. Instead of (with-decorator f (fn ...)), just say (f (fn ...)).

[pyx]

Fair enough, so the problem remaining is better error message.

[pyx]

Playing with it a bit, I think there is a discrepancy here.

How can we translate this Python code into Hy code?

import functools

def the_answer(f):
    @functools.wraps(f)
    def decorated():
        return 42
    return decorated

@the_answer
def tell_me():
    return 0

tell_me()  # -> 42

Literal translation (sans the return, which is not lispy) does not work here:

(import functools)

(defn the-answer [f]
  (with-decorator (functools.wraps f)
    (defn decorated []
      42)))

(with-decorator the-answer
  (defn tell-me [] 0))

(tell-me)  ;; raises "TypeError: 'NoneType' object is not callable"

I know sometime ago, defn had been re-designed as returning None, combining this fact with the current behaviour of with-decorator, it breaks the "Hy <-> Python interop" to certain extent.

Maybe the true literal translation with (return decorated) is what's preferred?

[Kodiologist]

Yeah, you need to put decorated, or equivalently (return decorated), at the end of the definition of the-answer.with-decorator returns None, not the decorated function. If you want to be more concise, you can write

(defn the-answer [f]
  ((functools.wraps f) (fn [] 42)))

[pyx]

docmebro, maybe? Cause the proposed way is not lispy (for my own definition of lispy, that is 😄).
I know with-decorator support more than one decorators, but returning None makes nesting with-decorator impossible (not that I think that is a good idea).

[Kodiologist]

returning None makes nesting with-decorator impossible (not that I think that is a good idea).

No, nesting with-decorator still works:

(defn dec1 [f]
  (fn []
    (print "in dec1")
    (f)))

(defn dec2 [f]
  (fn []
    (print "in dec2")
    (f)))

(with-decorator dec1
  (with-decorator dec2
    (defn foo []
      42)))

(print (foo))

This prints:

in dec1
in dec2
42

[pyx]

My fault. I mistakenly thought that because with-decorator evaluated to None, nesting does not work. Didn't know that it is a special form on AST level until I just checked hy.compiler.py, I thought it was either on macro's level or run-time level.

Thanks for your clarification.

[Kodiologist]

Right, it has to be a special form because it directly modifies attributes of AST nodes.