Better way to type Coconut data types

[ArneBachmann]

I wanted to do something like A = TypeVar("A", bound = DA) where data DA(a:int, b:str) but it wasn't allowed to reference the _coconut definitions

[evhub]

@ArneBachmann Can you give me a specific piece of code that produces the error you're encountering?

[ArneBachmann]

Actual use case was a copy-constructor for data types. This didn't work:

from typing import Type, TypeVar
data BranchInfo(number:int, ctime:int, name:str? = None, tracked:str[] = [])
try: BI = TypeVar("BI", bound = BranchInfo)  # only valid for Python 3
except: pass
def dataCopy(tipe:Type[BI], old:BI, **kwargs): r = old._asdict(); r.update(**kwargs); return tipe(**r)

Unsupported type Type["BranchInfo"]

but this did:

from typing import Type, TypeVar
data BranchInfo(number:int, ctime:int, name:str? = None, tracked:str[] = [])
try: BI = TypeVar("BI", bound = BranchInfo)  # only valid for Python 3
except: pass
def dataCopy(tipe:Type[BI], old:BI, **kwargs): r = old._asdict(); r.update(**kwargs); return makedata(tipe, **r)

<no mypy complaints>

The only change is using makedata instead of calling the constructor.

But in this code, I have to pin the copy function to a certain data type, instead of to generic data types.

[ArneBachmann]

Found a generic workaround that doesn't complain in mypy and runs with any namedtuple (problem is that namedtuple is a runtime code-generator factory):

import collections
from typing import Type, TypeVar

class _AsDict(tuple):
  def _asdict(_) -> collections.OrderedDict: pass  # type hinting for mypy

try: DataType = TypeVar("DataType", bound = _AsDict)
except: pass  # Python 2 mypy

def dataCopy(tipe:Type[DataType], old:DataType, **kwargs) =
  r = old._asdict()
  r.update(**kwargs)
  makedata(tipe, *[r[field] for field in old._fields])  # **r doesn't work, only positional arguments allowed :-(

[ArneBachmann]

Exposing something like above _AsDict as CoconutDataType and/or coconut_typing.DataType would close probably this issue.

[ArneBachmann]

Current solution focusing purely on Python 3:

from typing import Type, TypeVar
DataType = TypeVar("DataType", MergeBlock, BranchInfo)  # list all your types here
def dataCopy(_tipe:Type[DataType], _old:DataType, *_args, **_kwargs) -> DataType: r = _old._asdict(); r.update(**_kwargs); return makedata(_tipe, *(list(_args) + [r[field] for field in _old._fields]))

[evhub]

@ArneBachmann I see what's going on here. What you should do is just use typing.NamedTuple exactly how you would use a CoconutDataType. I'm thinking one nice way to resolve this might be to support turning the keyword data in a type definition into _coconut_NamedTuple, which would fix the issue.

[ArneBachmann]

If every data type was a namedtuple, it might work, is that what you mean as well? I'm looking for a way to type check data/named tuples generically as seen in the function above

[evhub]

@ArneBachmann From MyPy's perspective, all data types should look like named tuples.

[ArneBachmann]

OK I don't get it. Can you provide a minimal example on how to typecheck them datat ypes in Coco --mypy?

[evhub]

@ArneBachmann For example:

~\Documents\GitHub> co --mypy
Coconut Interpreter:
(type 'exit()' or press Ctrl-D to end)
>>> try: from typing import NamedTuple
    except: pass

>>> data vec(x, y)
>>> data intvec(x: int, y: int)
>>> def scale(s: int, v: NamedTuple) = v |> fmap$(-> _*s)
>>> vec(1, 1) |> scale$(2)
vec(x=2, y=2)
>>> intvec(1, 1) |> scale$(2)
intvec(x=2, y=2)

As you can see, both untyped (vec) and typed (intvec) data types are, for the purposes of type-checking, of type typing.NamedTuple. Note that the typing import need not succeed for this to work--MyPy will still know what you mean even if it doesn't.

[ArneBachmann]

That doesn´t work... I cannot simply replace any data type with a NamedTuple. Also I want to ensure the function´s output is the same NamedTuple type, not just any one.

error: Incompatible types in assignment (expression has type "NamedTuple", variable has type "ChangeSet")

[evhub]

@ArneBachmann Not sure what changed, but it looks like this is just working now, so I'm closing this issue:

> co -t sys --mypy                                                                            Coconut Interpreter:
(enter 'exit()' or press Ctrl-D to end)
>>> data vec(x, y)
Success: no issues found in 1 source file
>>> def f(v: vec) = v
Success: no issues found in 1 source file
>>> f(vec(1, 2))
vec(x=1, y=2)
Success: no issues found in 1 source file
>>>