Hello, On Fri, 22 May 2020 21:01:03 +0100 Andrey Cizov <aci...@gmail.com> wrote:
> Sorry I forgot to add the URL: > https://pypi.org/project/tagged-dataclasses/ How does this compare with many other implementations spread over the interwebs? As a quick comment, looks verbose comparing to ML ;-). For comparison, I found an algebraic2.py in ~/my-python-hacks/, no dataclasses or particular IDEs involved: -------------- from collections import namedtuple # Ideal syntax: # #Type = ( # Int(i: int) | # Str(s: str) | # Plus(l: Type, r: Type) #) def V(name, **args): return namedtuple(name, args.keys()) class Union: def __init__(self, *variants): self.variants = list(variants) def add(self, *variants): self.variants.extend(variants) def __instancecheck__(self, inst): return isinstance(inst, tuple(self.variants)) def match(val, typ): if isinstance(val, typ): # Have to return scalar instead of a tuple due to CPython # deficiency with := operator return val[0] -------------- Can be used as: -------------- UnaryExpr = Union( Int := V("Int", i=int), Str := V("Str", s=str), ) # Recursive variants should be added after initial definition UnaryExpr.add( Inc := V("Inc", e=UnaryExpr), Dec := V("Dec", e=UnaryExpr), ) def eval(var: UnaryExpr): if i := match(var, Int): return i elif s := match(var, Str): return s elif e := match(var, Inc): return eval(e) + 1 elif e := match(var, Dec): return eval(e) - 1 expr = Dec(Int(123)) print(isinstance(expr, UnaryExpr)) print(eval(expr)) -------------- It's sad "UnaryExpr" (instead of e.g. BinaryExpr) because of a known deficiency of CPython: --- $ python3.8 -c "( (a, b) := (1, 2) )" File "<string>", line 1 SyntaxError: cannot use assignment expressions with tuple --- Btw, does anybody know a Python implementation which has this bug fixed? > > On Fri, 22 May 2020 at 20:25, Andrey Cizov <aci...@gmail.com> wrote: > > > I have developed a library to introduce tagged unions to python > > that uses dataclasses to define disjoint members of the tagged > > union (by defining them as Optional fields). With some additional > > validation I make sure that only one of the fields is not None. > > > > I find that it also fits well with the existing analysis tools and > > IDEs (e.g. PyCharm) and doesn’t require any additional work in > > order to be supported. > > > > I would like to see some criticism and whether that could > > potentially be a good candidate for python standard library in the > > future. [] -- Best regards, Paul mailto:pmis...@gmail.com _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-le...@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/CVOCMSJYQHTUADG5GEQICVV3X25JLR5R/ Code of Conduct: http://python.org/psf/codeofconduct/
