[Python-ideas] Re: More natural type hints for built-in containers

2 Aug 2021


      Yeah just switch between case and match.
def func(x, alist, z):  # Type-hints remain optional.
       """Docstring goes here"""
       match (x, alist, z):
           case (int(x), [float(y), _], 0):
               return 0
           case (int(x), [float(y), _], int(z)):
               return int((x+y)/z)
           case _:
               raise SomeException

Why don’t we just remove the match inside a function body if we are matching the function parameters? This way, we don’t repeat the parameters again and remove one level of indentation. So the rule like this, if the cases are inside the function, it’s implied we are pattern matching the parameters if we don’t specify the “match” statement.  Of course if you opt to use the match statement, you can pattern match anything including the parameters themselves like the example above.
...
On 2 Aug 2021, at 4:39 AM, Steven D'Aprano  wrote:
On Sun, Aug 01, 2021 at 07:42:07PM +0400, Abdulla Al Kathiri wrote:
...
While at it, why don’t we pattern match function parameters like 
Haskel. Very elegant way for function overload.
I don't think your example is very elegant:
...
case def func(int(x), [float(y), *_], int(z)) if z != 0 -> int:
       return int((x + y) / z)
case def func(int(x), [float(y), *_], int(z)): -> int:
       return 0
case def func(_):
       raise
That's a clear violation of DRY. The indentation is odd. And where does 
the docstring go? What will `func.__annotations__` be?
It is especially awkward if you have a realistic function body rather 
than a single line statement, and the syntax is just begging to be 
mis-written as:
case def func(...):
       body
case def func(...):
       body
def func(...):
       body
especially if the bodies are realistically large.
...
No need to precede it with the “match” statement because we are 
pattern matching function parameters here in those cases. Also, 
annotating the parameters will be redundant.
What is so special about the case where you are matching on *exactly* 
the parameters as written? That may be common, but I don't think that it 
is so common that we need special syntax for it. A more general function 
would pattern match on only some of the parameters, with the other 
parameters used elsewhere in the function.
Of course we can always just match the other parameters with the 
wild-card, but that's not very elegent.
It also promotes function annotations from an optional type hint with 
no runtime semantics (other than setting `__annotations__`) to syntactic 
sugar for runtime function multiple dispatch.
I think that would be far better written as a single function with the 
case inside the body.
# I may have the syntax of the case statement wrong...
   def func(x, alist, z):  # Type-hints remain optional.
       """Docstring goes here"""
       case (x, alist, z):
           match (int(x), [float(y), _], 0):
               return 0
           match (int(x), [float(y), _], int(z)):
               return int((x+y)/z)
           match _:
               raise SomeException
-- 
Steve
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