This is a tag to discuss the Union Type feature as seen in languages such as TypeScript, Ceylon or F#.
Questions tagged [union-types]
277 questions
96
votes
5 answers
What does "keyof typeof" mean in TypeScript?
Example:
Explain to me what keyof typeof means in TypeScript
enum ColorsEnum {
white = '#ffffff',
black = '#000000',
}
type Colors = keyof typeof ColorsEnum;
The last row is equivalent to:
type Colors = "white" | "black"
But how does it…
user1283776
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61
votes
3 answers
mypy, type hint: Union[float, int] -> is there a Number type?
mypy is really handy and catches a lot of bugs, but when I write "scientific" applications, I often end up doing:
def my_func(number: Union[float, int]):
# Do something
number is either a float or int, depending on the user's input. Is there an…
JPFrancoia
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40
votes
2 answers
How to define string literal union type from constants in Typescript
I know I can define string union types to restrict variables to one of the possible string values:
type MyType = 'first' | 'second'
let myVar:MyType = 'first'
I need to construct a type like that from constant strings, e.g:
const MY_CONSTANT =…
Can Poyrazoğlu
- 29,145
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17
votes
1 answer
What's the difference between a constrained TypeVar and a Union?
If I want to have a type that can be multiple possible types, Unions seem to be how I represent that:
U = Union[int, str]
U can be an int or a str.
I noticed though that TypeVars allow for optional var-arg arguments that also seem to do the same…
Carcigenicate
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15
votes
2 answers
PHP Type hinting to allow Array or ArrayAccess
Is it possible to allow an Array or an object that implements ArrayAccess?
For example:
class Config implements ArrayAccess {
...
}
class I_Use_A_Config
{
public function __construct(Array $test)
...
}
I want to be able to pass in…
Supericy
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12
votes
1 answer
How to extract a type from a tagged union type in typescript?
Say there is already a type defined as this:
export type Item = {
type: 'text',
content: string
} | {
type: 'link',
url: string
}
Is it possible to extract the link part from the type Item? I mean, is it possible to define a type…
Freewind
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10
votes
2 answers
How to type a Typescript array to accept only a specific set of values?
I am writing a type declaration file for a library I do not control. One of the methods accepts an array of strings as a parameter, but these strings can only be very specific values. Currently I am typing this parameter as a string[], but I was…
Shaun Hamman
- 1,947
- 3
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- 33
9
votes
4 answers
Algebraic Data Types in Postgres
Is it possible to create an Algebraic Data Type in Postgres and then use it as a column type?
For example:
CREATE TYPE hoofed AS ENUM('horse', 'goat');
CREATE TYPE monkey AS ENUM('chimp','macaque');
CREATE TYPE ANIMAL AS ENUM(hoofed,…
Abraham P
- 13,493
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- 108
8
votes
4 answers
Mixing union types, generics and conditional types causes unexpected "Type is not assignable to type" error
I've hit a problem with type-inference specifically when conditional-types are used within union types.
There may be a shorter way to demonstrate this issue, but I could not find one...
See the problem in action at this playground link.
Consider the…
spender
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8
votes
1 answer
Elm Tagged Union Type compare constructor
Is it possible to use == to compare the constructor of tagged union types in Elm, or do you have to use case?
Example:
type alias Model =
{ accessToken : String
, page : Page
, config : Config
}
type Page
= Home String
…
Xyking
- 83
- 4
7
votes
1 answer
How does Dotty decide how to infer/when to widen Union Types?
Widening Union Types have been discussed here but I can't seem to find an answer to the following case
Let's start by looking at the following
val x = List(1, 2, "a")
This heterogeneous list is inferred as List[Any] Just like it would in Scala…
sinanspd
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7
votes
2 answers
"Unsupported target for indexed assignment" with mypy, depending on type hinting moment with respect to assignment
I'm trying to do some typing on my python code, and I got the following mypy error: "Unsupported target for indexed assignment"
On a simplified example, it amounts to the following code:
from pathlib import Path
from typing import (Literal,…
bli
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7
votes
1 answer
TS2339: Property does not exist on union type - property string | undefined
I have problem with my union type, which looks like this:
type RepeatForm = {
step:
| {
repeat: false;
}
| {
repeat: true;
from: undefined;
}
| {
…
Konrad Klimczak
- 1,047
- 2
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6
votes
0 answers
Generic union type inferred differently from equivalent direct union type
Consider the following:
type UnwrapNullable1 = Required<{ x?: (1 | undefined) }>["x"]
type UnwrapNullable = Required<{ x?: T }>["x"]
type Test = UnwrapNullable<1 | undefined>
The type UnwrapNullable1 is correctly inferred as 1, but Test, which…
prmph
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6
votes
3 answers
Typescript - how to combine Union and Intersection types
I have the following component:
export enum Tags {
button = 'button',
a = 'a',
input = 'input',
}
type ButtonProps = {
tag: Tags.button;
} & ({ a?: string; b?: undefined } | { a?: undefined; b?: string }) &
…
leepowell
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