This tutorial will guide you through the process of creating your own containers.
First things first, why would anyone want to create a custom containers?
The great idea about “containers” in functional programming is that it can be literally anything. There are endless use-cases.
You can create your own primitives for working with some language-or-framework specific problem, or just model your business domain.
You can copy ideas from other languages or just compose existing containers
for better usability
(like IOResult
is the composition of IO
and Result
).
Example
We are going to implement a Pair
container for this example.
What is a Pair
? Well, it is literally a pair of two values.
No more, no less. Similar to a Tuple[FirstType, SecondType]
.
But with extra goodies.
Note
You can find all code samples here.
After you came up with the idea, you will need to make a decision: what capabilities my container must have?
Basically, you should decide what Interfaces you will subtype and what
methods and laws will be present in your type.
You can create just a returns.interfaces.mappable.MappableN
or choose a full featured returns.interfaces.container.ContainerN
.
You can also choose some specific interfaces to use,
like returns.interfaces.specific.result.ResultLikeN
or any other.
Summing up, decide what laws and methods you need to solve your problem. And then subtype the interfaces that provide these methods and laws.
Example
What interfaces a Pair
type needs?
returns.interfaces.equable.Equable
,
because two Pair
instances can be compared
returns.interfaces.mappable.MappableN
,
because the first type can be composed with pure functions
returns.interfaces.bindable.BindableN
,
because a Pair
can be bound to a function returning a new Pair
based on the first type
returns.interfaces.altable.AltableN
,
because the second type can be composed with pure functions
returns.interfaces.lashable.LashableN
,
because a Pair
can be bound to a function returning a new Pair
based on the second type
Now, after we know about all interfaces we would need, let’s find pre-defined aliases we can reuse.
Turns out, there are some of them!
returns.interfaces.bimappable.BiMappableN
which combines MappableN
and AltableN
returns.interfaces.swappable.SwappableN
is an alias for BiMappableN
with a new method called .swap
to change values order
Let’s look at the result:
Note
A special note on returns.primitives.container.BaseContainer
.
It is a very useful class with lots of pre-defined feaatures, like:
immutability, better cloning, serialization, and comparison.
You can skip it if you wish, but it is highlighly recommended.
Later we will talk about an actual implementation of all required methods.
So, let’s start writing some code!
We would need to implement all interface methods,
otherwise mypy
won’t be happy.
That’s what it currently says on our type definition:
error: Final class test_pair1.Pair has abstract attributes "alt", "bind", "equals", "lash", "map", "swap"
Looks like it already knows what methods should be there!
Ok, let’s drop some initial and straight forward implementation. We will later make it more complex step by step.
1from typing import Callable, Tuple, TypeVar
2
3from typing_extensions import final
4
5from returns.interfaces import bindable, equable, lashable, swappable
6from returns.primitives.container import BaseContainer, container_equality
7from returns.primitives.hkt import Kind2, SupportsKind2, dekind
8
9_FirstType = TypeVar('_FirstType')
10_SecondType = TypeVar('_SecondType')
11
12_NewFirstType = TypeVar('_NewFirstType')
13_NewSecondType = TypeVar('_NewSecondType')
14
15
16@final
17class Pair(
18 BaseContainer,
19 SupportsKind2['Pair', _FirstType, _SecondType],
20 bindable.Bindable2[_FirstType, _SecondType],
21 swappable.Swappable2[_FirstType, _SecondType],
22 lashable.Lashable2[_FirstType, _SecondType],
23 equable.Equable,
24):
25 """
26 A type that represents a pair of something.
27
28 Like to coordinates ``(x, y)`` or two best friends.
29 Or a question and an answer.
30
31 """
32
33 def __init__(
34 self,
35 inner_value: Tuple[_FirstType, _SecondType],
36 ) -> None:
37 """Saves passed tuple as ``._inner_value`` inside this instance."""
38 super().__init__(inner_value)
39
40 # `Equable` part:
41
42 equals = container_equality # we already have this defined for all types
43
44 # `Mappable` part via `BiMappable`:
45
46 def map(
47 self,
48 function: Callable[[_FirstType], _NewFirstType],
49 ) -> 'Pair[_NewFirstType, _SecondType]':
50 return Pair((function(self._inner_value[0]), self._inner_value[1]))
51
52 # `BindableN` part:
53
54 def bind(
55 self,
56 function: Callable[
57 [_FirstType],
58 Kind2['Pair', _NewFirstType, _SecondType],
59 ],
60 ) -> 'Pair[_NewFirstType, _SecondType]':
61 return dekind(function(self._inner_value[0]))
62
63 # `AltableN` part via `BiMappableN`:
64
65 def alt(
66 self,
67 function: Callable[[_SecondType], _NewSecondType],
68 ) -> 'Pair[_FirstType, _NewSecondType]':
69 return Pair((self._inner_value[0], function(self._inner_value[1])))
70
71 # `LashableN` part:
72
73 def lash(
74 self,
75 function: Callable[
76 [_SecondType],
77 Kind2['Pair', _FirstType, _NewSecondType],
78 ],
79 ) -> 'Pair[_FirstType, _NewSecondType]':
80 return dekind(function(self._inner_value[1]))
81
82 # `SwappableN` part:
83
84 def swap(self) -> 'Pair[_SecondType, _FirstType]':
85 return Pair((self._inner_value[1], self._inner_value[0]))
You can check our resulting source with mypy
. It would be happy this time.
As you can see our existing interfaces do not cover everything. We can potentially want several extra things:
A method that takes two arguments and returns a new Pair
instance
A named constructor to create a Pair
from a single value
A named constructor to create a Pair
from two values
We can define an interface just for this! It would be also nice to add all other interfaces there as supertypes.
That’s how it is going to look:
1class PairLikeN(
2 bindable.BindableN[_FirstType, _SecondType, _ThirdType],
3 swappable.SwappableN[_FirstType, _SecondType, _ThirdType],
4 lashable.LashableN[_FirstType, _SecondType, _ThirdType],
5 equable.Equable,
6):
7 """Special interface for types that look like a ``Pair``."""
8
9 @abstractmethod
10 def pair(
11 self: _PairLikeKind,
12 function: Callable[
13 [_FirstType, _SecondType],
14 KindN[_PairLikeKind, _NewFirstType, _NewSecondType, _ThirdType],
15 ],
16 ) -> KindN[_PairLikeKind, _NewFirstType, _NewSecondType, _ThirdType]:
17 """Allows to work with both arguments at the same time."""
18
19 @classmethod
20 @abstractmethod
21 def from_paired(
22 cls: Type[_PairLikeKind],
23 first: _NewFirstType,
24 second: _NewSecondType,
25 ) -> KindN[_PairLikeKind, _NewFirstType, _NewSecondType, _ThirdType]:
26 """Allows to create a PairLikeN from just two values."""
27
28 @classmethod
29 @abstractmethod
30 def from_unpaired(
31 cls: Type[_PairLikeKind],
32 inner_value: _NewFirstType,
33 ) -> KindN[_PairLikeKind, _NewFirstType, _NewFirstType, _ThirdType]:
34 """Allows to create a PairLikeN from just a single object."""
Awesome! Now we have a new interface to implement. Let’s do that!
1 def pair(
2 self,
3 function: Callable[
4 [_FirstType, _SecondType],
5 Kind2['Pair', _NewFirstType, _NewSecondType],
6 ],
7 ) -> 'Pair[_NewFirstType, _NewSecondType]':
8 return dekind(function(self._inner_value[0], self._inner_value[1]))
1 @classmethod
2 def from_unpaired(
3 cls,
4 inner_value: _NewFirstType,
5 ) -> 'Pair[_NewFirstType, _NewFirstType]':
6 return Pair((inner_value, inner_value))
Looks like we are done!
The best part about this type is that it is pure. So, we can write our tests inside docs!
We are going to use doctests builtin module for that.
This gives us several key benefits:
All our docs has usage examples
All our examples are correct, because they are executed and tested
We don’t need to write regular boring tests
Let’s add docs and doctests! Let’s use map
method as a short example:
1 def map(
2 self,
3 function: Callable[[_FirstType], _NewFirstType],
4 ) -> 'Pair[_NewFirstType, _SecondType]':
5 """
6 Changes the first type with a pure function.
7
8 >>> assert Pair((1, 2)).map(str) == Pair(('1', 2))
9
10 """
11 return Pair((function(self._inner_value[0]), self._inner_value[1]))
By adding these simple tests we would already have 100% coverage. But, what if we can completely skip writing tests, but still have 100%?
Let’s discuss how we can achieve that with “Laws as values”.
We already ship lots of laws with our interfaces. See our docs on laws and checking them.
Moreover, you can also define your own laws!
Let’s add them to our PairLikeN
interface.
Let’s start with laws definition:
1class _LawSpec(LawSpecDef):
2 @law_definition
3 def pair_equality_law(
4 raw_value: _FirstType,
5 container: 'PairLikeN[_FirstType, _SecondType, _ThirdType]',
6 ) -> None:
7 """Ensures that unpaired and paired constructors work fine."""
8 assert_equal(
9 container.from_unpaired(raw_value),
10 container.from_paired(raw_value, raw_value),
11 )
12
13 @law_definition
14 def pair_left_identity_law(
15 pair: Tuple[_FirstType, _SecondType],
16 container: 'PairLikeN[_FirstType, _SecondType, _ThirdType]',
17 function: Callable[
18 [_FirstType, _SecondType],
19 KindN['PairLikeN', _NewFirstType, _NewSecondType, _ThirdType],
20 ],
21 ) -> None:
22 """Ensures that unpaired and paired constructors work fine."""
23 assert_equal(
24 container.from_paired(*pair).pair(function),
25 function(*pair),
26 )
And them let’s add them to our PairLikeN
interface:
1class PairLikeN(
2 bindable.BindableN[_FirstType, _SecondType, _ThirdType],
3 swappable.SwappableN[_FirstType, _SecondType, _ThirdType],
4 lashable.LashableN[_FirstType, _SecondType, _ThirdType],
5 equable.Equable,
6):
7 """Special interface for types that look like a ``Pair``."""
8
9 _laws: ClassVar[Sequence[Law]] = (
10 Law2(_LawSpec.pair_equality_law),
11 Law3(_LawSpec.pair_left_identity_law),
12 )
13
14 @abstractmethod
15 def pair(
16 self: _PairLikeKind,
17 function: Callable[
18 [_FirstType, _SecondType],
19 KindN[_PairLikeKind, _NewFirstType, _NewSecondType, _ThirdType],
20 ],
21 ) -> KindN[_PairLikeKind, _NewFirstType, _NewSecondType, _ThirdType]:
22 """Allows to work with both arguments at the same time."""
23
24 @classmethod
25 @abstractmethod
26 def from_paired(
27 cls: Type[_PairLikeKind],
28 first: _NewFirstType,
29 second: _NewSecondType,
30 ) -> KindN[_PairLikeKind, _NewFirstType, _NewSecondType, _ThirdType]:
31 """Allows to create a PairLikeN from just two values."""
32
33 @classmethod
34 @abstractmethod
35 def from_unpaired(
36 cls: Type[_PairLikeKind],
37 inner_value: _NewFirstType,
38 ) -> KindN[_PairLikeKind, _NewFirstType, _NewFirstType, _ThirdType]:
39 """Allows to create a PairLikeN from just a single object."""
The last to do is to call check_all_laws(Pair, use_init=True)
to generate 10 hypothesis
test cases with hundreds real test cases inside.
Here’s the final result of our brand new Pair
type:
1from abc import abstractmethod
2from typing import Callable, ClassVar, NoReturn, Sequence, Tuple, Type, TypeVar
3
4from typing_extensions import final
5
6from returns.contrib.hypothesis.laws import check_all_laws
7from returns.interfaces import bindable, equable, lashable, swappable
8from returns.primitives.asserts import assert_equal
9from returns.primitives.container import BaseContainer, container_equality
10from returns.primitives.hkt import Kind2, KindN, SupportsKind2, dekind
11from returns.primitives.laws import Law, Law2, Law3, LawSpecDef, law_definition
12
13_FirstType = TypeVar('_FirstType')
14_SecondType = TypeVar('_SecondType')
15_ThirdType = TypeVar('_ThirdType')
16
17_NewFirstType = TypeVar('_NewFirstType')
18_NewSecondType = TypeVar('_NewSecondType')
19
20_PairLikeKind = TypeVar('_PairLikeKind', bound='PairLikeN')
21
22
23class _LawSpec(LawSpecDef):
24 @law_definition
25 def pair_equality_law(
26 raw_value: _FirstType,
27 container: 'PairLikeN[_FirstType, _SecondType, _ThirdType]',
28 ) -> None:
29 """Ensures that unpaired and paired constructors work fine."""
30 assert_equal(
31 container.from_unpaired(raw_value),
32 container.from_paired(raw_value, raw_value),
33 )
34
35 @law_definition
36 def pair_left_identity_law(
37 pair: Tuple[_FirstType, _SecondType],
38 container: 'PairLikeN[_FirstType, _SecondType, _ThirdType]',
39 function: Callable[
40 [_FirstType, _SecondType],
41 KindN['PairLikeN', _NewFirstType, _NewSecondType, _ThirdType],
42 ],
43 ) -> None:
44 """Ensures that unpaired and paired constructors work fine."""
45 assert_equal(
46 container.from_paired(*pair).pair(function),
47 function(*pair),
48 )
49
50
51class PairLikeN(
52 bindable.BindableN[_FirstType, _SecondType, _ThirdType],
53 swappable.SwappableN[_FirstType, _SecondType, _ThirdType],
54 lashable.LashableN[_FirstType, _SecondType, _ThirdType],
55 equable.Equable,
56):
57 """Special interface for types that look like a ``Pair``."""
58
59 _laws: ClassVar[Sequence[Law]] = (
60 Law2(_LawSpec.pair_equality_law),
61 Law3(_LawSpec.pair_left_identity_law),
62 )
63
64 @abstractmethod
65 def pair(
66 self: _PairLikeKind,
67 function: Callable[
68 [_FirstType, _SecondType],
69 KindN[_PairLikeKind, _NewFirstType, _NewSecondType, _ThirdType],
70 ],
71 ) -> KindN[_PairLikeKind, _NewFirstType, _NewSecondType, _ThirdType]:
72 """Allows to work with both arguments at the same time."""
73
74 @classmethod
75 @abstractmethod
76 def from_paired(
77 cls: Type[_PairLikeKind],
78 first: _NewFirstType,
79 second: _NewSecondType,
80 ) -> KindN[_PairLikeKind, _NewFirstType, _NewSecondType, _ThirdType]:
81 """Allows to create a PairLikeN from just two values."""
82
83 @classmethod
84 @abstractmethod
85 def from_unpaired(
86 cls: Type[_PairLikeKind],
87 inner_value: _NewFirstType,
88 ) -> KindN[_PairLikeKind, _NewFirstType, _NewFirstType, _ThirdType]:
89 """Allows to create a PairLikeN from just a single object."""
90
91
92PairLike2 = PairLikeN[_FirstType, _SecondType, NoReturn]
93PairLike3 = PairLikeN[_FirstType, _SecondType, _ThirdType]
94
95
96@final
97class Pair(
98 BaseContainer,
99 SupportsKind2['Pair', _FirstType, _SecondType],
100 PairLike2[_FirstType, _SecondType],
101):
102 """
103 A type that represents a pair of something.
104
105 Like to coordinates ``(x, y)`` or two best friends.
106 Or a question and an answer.
107
108 """
109
110 def __init__(
111 self,
112 inner_value: Tuple[_FirstType, _SecondType],
113 ) -> None:
114 """Saves passed tuple as ``._inner_value`` inside this instance."""
115 super().__init__(inner_value)
116
117 # `Equable` part:
118
119 equals = container_equality # we already have this defined for all types
120
121 # `Mappable` part via `BiMappable`:
122
123 def map(
124 self,
125 function: Callable[[_FirstType], _NewFirstType],
126 ) -> 'Pair[_NewFirstType, _SecondType]':
127 """
128 Changes the first type with a pure function.
129
130 >>> assert Pair((1, 2)).map(str) == Pair(('1', 2))
131
132 """
133 return Pair((function(self._inner_value[0]), self._inner_value[1]))
134
135 # `BindableN` part:
136
137 def bind(
138 self,
139 function: Callable[
140 [_FirstType],
141 Kind2['Pair', _NewFirstType, _SecondType],
142 ],
143 ) -> 'Pair[_NewFirstType, _SecondType]':
144 """
145 Changes the first type with a function returning another ``Pair``.
146
147 >>> def bindable(first: int) -> Pair[str, str]:
148 ... return Pair((str(first), ''))
149
150 >>> assert Pair((1, 'b')).bind(bindable) == Pair(('1', ''))
151
152 """
153 return dekind(function(self._inner_value[0]))
154
155 # `AltableN` part via `BiMappableN`:
156
157 def alt(
158 self,
159 function: Callable[[_SecondType], _NewSecondType],
160 ) -> 'Pair[_FirstType, _NewSecondType]':
161 """
162 Changes the second type with a pure function.
163
164 >>> assert Pair((1, 2)).alt(str) == Pair((1, '2'))
165
166 """
167 return Pair((self._inner_value[0], function(self._inner_value[1])))
168
169 # `LashableN` part:
170
171 def lash(
172 self,
173 function: Callable[
174 [_SecondType],
175 Kind2['Pair', _FirstType, _NewSecondType],
176 ],
177 ) -> 'Pair[_FirstType, _NewSecondType]':
178 """
179 Changes the second type with a function returning ``Pair``.
180
181 >>> def lashable(second: int) -> Pair[str, str]:
182 ... return Pair(('', str(second)))
183
184 >>> assert Pair(('a', 2)).lash(lashable) == Pair(('', '2'))
185
186 """
187 return dekind(function(self._inner_value[1]))
188
189 # `SwappableN` part:
190
191 def swap(self) -> 'Pair[_SecondType, _FirstType]':
192 """
193 Swaps ``Pair`` elements.
194
195 >>> assert Pair((1, 2)).swap() == Pair((2, 1))
196
197 """
198 return Pair((self._inner_value[1], self._inner_value[0]))
199
200 # `PairLikeN` part:
201
202 def pair(
203 self,
204 function: Callable[
205 [_FirstType, _SecondType],
206 Kind2['Pair', _NewFirstType, _NewSecondType],
207 ],
208 ) -> 'Pair[_NewFirstType, _NewSecondType]':
209 """
210 Creates a new ``Pair`` from an existing one via a passed function.
211
212 >>> def min_max(first: int, second: int) -> Pair[int, int]:
213 ... return Pair((min(first, second), max(first, second)))
214
215 >>> assert Pair((2, 1)).pair(min_max) == Pair((1, 2))
216 >>> assert Pair((1, 2)).pair(min_max) == Pair((1, 2))
217
218 """
219 return dekind(function(self._inner_value[0], self._inner_value[1]))
220
221 @classmethod
222 def from_paired(
223 cls,
224 first: _NewFirstType,
225 second: _NewSecondType,
226 ) -> 'Pair[_NewFirstType, _NewSecondType]':
227 """
228 Creates a new pair from two values.
229
230 >>> assert Pair.from_paired(1, 2) == Pair((1, 2))
231
232 """
233 return Pair((first, second))
234
235 @classmethod
236 def from_unpaired(
237 cls,
238 inner_value: _NewFirstType,
239 ) -> 'Pair[_NewFirstType, _NewFirstType]':
240 """
241 Creates a new pair from a single value.
242
243 >>> assert Pair.from_unpaired(1) == Pair((1, 1))
244
245 """
246 return Pair((inner_value, inner_value))
247
248
249# Running hypothesis auto-generated tests:
250check_all_laws(Pair, use_init=True)
Note
You can find all type-tests here.
The next thing we want is to write a type-test!
What is a type-test? This is a special type of tests for your typing.
We run mypy
on top of tests and use snapshots to assert the result.
We recommend to use pytest-mypy-plugins. Read more about how to use it.
Let’s start with a simple test
to make sure our .pair
function works correctly:
Warning
Please, don’t use env:
property the way we do here.
We need it since we store our example in tests/
folder.
And we have to tell mypy
how to find it.
1- case: test_pair_type
2 disable_cache: false
3 env:
4 # We only need this because we store this example in `tests/`
5 # and not in our source code. Please, do not copy this line!
6 - MYPYPATH=./tests/test_examples/test_your_container
7 main: |
8 # Let's import our `Pair` type we defined earlier:
9 from test_pair4 import Pair
10
11 def function(first: int, second: str) -> Pair[float, bool]:
12 ...
13
14 my_pair: Pair[int, str] = Pair.from_paired(1, 'a')
15 reveal_type(my_pair.pair(function))
16 out: |
17 main:8: note: Revealed type is "test_pair4.Pair[builtins.float*, builtins.bool*]"
Ok, now, let’s try to raise an error by using it incorrectly:
1- case: test_pair_error
2 disable_cache: false
3 env:
4 # We only need this because we store this example in `tests/`
5 # and not in our source code. Please, do not copy this line!
6 - MYPYPATH=./tests/test_examples/test_your_container
7 main: |
8 # Let's import our `Pair` type we defined earlier:
9 from test_pair4 import Pair
10
11 # Oups! This function has first and second types swapped!
12 def function(first: str, second: int) -> Pair[float, bool]:
13 ...
14
15 my_pair = Pair.from_paired(1, 'a')
16 my_pair.pair(function) # this should and will error
17 out: |
18 main:9: error: Argument 1 to "pair" of "Pair" has incompatible type "Callable[[str, int], Pair[float, bool]]"; expected "Callable[[int, str], KindN[Pair[Any, Any], float, bool, Any]]"
The last (but not the least!) thing you need
to know is that you can reuse all code
we already have for this new Pair
type.
This is because of our Higher Kinded Types feature.
So, let’s say we want to use native map_()
pointfree function with our new Pair
type.
Let’s test that it will work correctly:
1- case: test_pair_map
2 disable_cache: false
3 env:
4 # We only need this because we store this example in `tests/`
5 # and not in our source code. Please, do not copy this line!
6 - MYPYPATH=./tests/test_examples/test_your_container
7 main: |
8 from test_pair4 import Pair
9 from returns.pointfree import map_
10
11 my_pair: Pair[int, int] = Pair.from_unpaired(1)
12 reveal_type(my_pair.map(str))
13 reveal_type(map_(str)(my_pair))
14 out: |
15 main:5: note: Revealed type is "test_pair4.Pair[builtins.str*, builtins.int]"
16 main:6: note: Revealed type is "test_pair4.Pair[builtins.str, builtins.int]"
Yes, it works!
Now you have fully working, typed, documented, lawful, and tested primitive. You can build any other primitive you need for your business logic or infrastructure.