[Python-ideas] ML Style Pattern Matching for Python

[spir]

On Sat, 18 Dec 2010 14:31:12 +0100
Eike Welk <eike.welk at gmx.net> wrote:

> Great that you thought about a syntax for dicts and sets.
> 
> 
> One other point that must be solved, is how to distinguish symbols that should 
> be assigned, from symbols that are specify conditions. Ocamls solution is that 
> all symbols are assigned. Conditions must be specified with literals. Here is 
> an example to illustrate it:
> 
> 
> This is a simple function that compares its argument "x" to the integer one:
> 
> def foo(x):
>     match x with
>     | 1 ->
>         print("x is equal to 1")
>     | _ ->
>         print("x is not equal to 1")
>         
>         
> In the code below we want to parametrize the value that we use for comparison. 
> The parameter "a" is introduced for this purpose. Now we need a way to tell 
> the compiler that "x" should be compared to "a"; and not assigned to "a". In 
> Ocaml this is impossible. The compiler would issue a warning ("Redundant case 
> in a pattern matching.").
> 
> def foo(x, a):
>     match x with
>     | a ->
>         print("x is equal to %s" % a)
>     | _ ->
>         print("x is not equal to %s" % a)
>         
> Maybe you come up with a nice notation for this problem. 

No, I would do it like in OCaml, I think (but may be wrong) it fits the python-way, like having no formally qualified readonly or private slots.

> By the way, why does no one like those "|" characters? They look good and take 
> up few space!

;-) It does not fit Python overall syntax look. And requires a magic id '_' for default case. And requires adding '->'. And we already have 'case x:' (and 'else') which fit perfectly. And are far more readable (no need to guess what given symbols mean, including the 'sense' of an arrow ;-).

> Eike.

Denis
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