Mixing
Unassignable function return type when generated by a generic function generator in Typescript
When I run this code:
type Converter<T extends ConverterOutput> = (value: string) => T
type ConverterOutput = number | string
function generateConverter<T extends ConverterOutput>(
value: string,
type: 'number' | 'string'
): Converter<T> {
return (value: string): T => {
switch (type) {
case 'number':
return Number(value)
default:
return value
}
}
}
I get these 2 errors:
[ts] Type 'number' is not assignable to type 'T'. [2322][ts] Type 'string' is not assignable to type 'T'. [2322]
for the 2 switch ... case returned values. I don't get what can be wrong in this code since T extends ConverterOutput which itself is a number | string.
I have tried adding a generic catch: return <U extends T>(value: string): U => { but it doesn't solve anything other than telling me that it's not assignable to U this time.
typescript
asked Nov 20 '18 at 14:51
Edouard HienrichsEdouard Hienrichs
38211
add a comment |
When I run this code:
type Converter<T extends ConverterOutput> = (value: string) => T
type ConverterOutput = number | string
function generateConverter<T extends ConverterOutput>(
value: string,
type: 'number' | 'string'
): Converter<T> {
return (value: string): T => {
switch (type) {
case 'number':
return Number(value)
default:
return value
}
}
}
I get these 2 errors:
[ts] Type 'number' is not assignable to type 'T'. [2322][ts] Type 'string' is not assignable to type 'T'. [2322]
for the 2 switch ... case returned values. I don't get what can be wrong in this code since T extends ConverterOutput which itself is a number | string.
I have tried adding a generic catch: return <U extends T>(value: string): U => { but it doesn't solve anything other than telling me that it's not assignable to U this time.
typescript
asked Nov 20 '18 at 14:51
Edouard HienrichsEdouard Hienrichs
38211
add a comment |
When I run this code:
type Converter<T extends ConverterOutput> = (value: string) => T
type ConverterOutput = number | string
function generateConverter<T extends ConverterOutput>(
value: string,
type: 'number' | 'string'
): Converter<T> {
return (value: string): T => {
switch (type) {
case 'number':
return Number(value)
default:
return value
}
}
}
I get these 2 errors:
[ts] Type 'number' is not assignable to type 'T'. [2322][ts] Type 'string' is not assignable to type 'T'. [2322]
for the 2 switch ... case returned values. I don't get what can be wrong in this code since T extends ConverterOutput which itself is a number | string.
I have tried adding a generic catch: return <U extends T>(value: string): U => { but it doesn't solve anything other than telling me that it's not assignable to U this time.
typescript
asked Nov 20 '18 at 14:51
Edouard HienrichsEdouard Hienrichs
38211
When I run this code:
type Converter<T extends ConverterOutput> = (value: string) => T
type ConverterOutput = number | string
function generateConverter<T extends ConverterOutput>(
value: string,
type: 'number' | 'string'
): Converter<T> {
return (value: string): T => {
switch (type) {
case 'number':
return Number(value)
default:
return value
}
}
}
I get these 2 errors:
[ts] Type 'number' is not assignable to type 'T'. [2322][ts] Type 'string' is not assignable to type 'T'. [2322]
for the 2 switch ... case returned values. I don't get what can be wrong in this code since T extends ConverterOutput which itself is a number | string.
I have tried adding a generic catch: return <U extends T>(value: string): U => { but it doesn't solve anything other than telling me that it's not assignable to U this time.
typescript
typescript
asked Nov 20 '18 at 14:51
Edouard HienrichsEdouard Hienrichs
38211
asked Nov 20 '18 at 14:51
Edouard HienrichsEdouard Hienrichs
38211
asked Nov 20 '18 at 14:51
Edouard HienrichsEdouard Hienrichs
38211
asked Nov 20 '18 at 14:51
Edouard HienrichsEdouard Hienrichs
38211
asked Nov 20 '18 at 14:51
Edouard HienrichsEdouard Hienrichs
38211
38211
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
Typescript will not let you assign concrete values where a value of a generic type parameter is expected. The reason for this is that usually the value can't be validated against all possible derived types that could conceivable be passed for the generic type parameter. For example even in your case, T extends number| string so this mean it could be the number literal type 1 which would not be satisfied by the value Number(value).
In your case I would recommend doing away with the generic type parameters and use multiple overloads, with the implementation signature returning Converter<number | string>. This has the added benefit that the type of the converter will only need to be specified as a string not as both string and type parameter.
type Converter<T extends ConverterOutput> = (value: string) => T
type ConverterOutput = number | string
function generateConverter(value: string, type: 'number'): Converter<number>
function generateConverter(value: string, type: 'string'): Converter<string>
function generateConverter(value: string, type: 'number' | 'string'): Converter<number | string> {
return (value) => {
switch (type) {
case 'number':
return Number(value)
default:
return value
}
}
}
answered Nov 20 '18 at 15:03
Titian Cernicova-DragomirTitian Cernicova-Dragomir
60.9k33755
add a comment |
Because a number nor a string does not extend itself. That's why it's not working. You can fix it by giving the type directly e.g.
function generateConverter(value: string, type: 'number' | 'string'): Converter<ConverterOutput> {
return (value: string): ConverterOutput => {
switch (type) {
case 'number':
return Number(value)
default:
return value
}
}
}
answered Nov 20 '18 at 15:03
Murat KaragözMurat Karagöz
14.5k53568
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
Typescript will not let you assign concrete values where a value of a generic type parameter is expected. The reason for this is that usually the value can't be validated against all possible derived types that could conceivable be passed for the generic type parameter. For example even in your case, T extends number| string so this mean it could be the number literal type 1 which would not be satisfied by the value Number(value).
In your case I would recommend doing away with the generic type parameters and use multiple overloads, with the implementation signature returning Converter<number | string>. This has the added benefit that the type of the converter will only need to be specified as a string not as both string and type parameter.
type Converter<T extends ConverterOutput> = (value: string) => T
type ConverterOutput = number | string
function generateConverter(value: string, type: 'number'): Converter<number>
function generateConverter(value: string, type: 'string'): Converter<string>
function generateConverter(value: string, type: 'number' | 'string'): Converter<number | string> {
return (value) => {
switch (type) {
case 'number':
return Number(value)
default:
return value
}
}
}
answered Nov 20 '18 at 15:03
Titian Cernicova-DragomirTitian Cernicova-Dragomir
60.9k33755
add a comment |
Typescript will not let you assign concrete values where a value of a generic type parameter is expected. The reason for this is that usually the value can't be validated against all possible derived types that could conceivable be passed for the generic type parameter. For example even in your case, T extends number| string so this mean it could be the number literal type 1 which would not be satisfied by the value Number(value).
In your case I would recommend doing away with the generic type parameters and use multiple overloads, with the implementation signature returning Converter<number | string>. This has the added benefit that the type of the converter will only need to be specified as a string not as both string and type parameter.
type Converter<T extends ConverterOutput> = (value: string) => T
type ConverterOutput = number | string
function generateConverter(value: string, type: 'number'): Converter<number>
function generateConverter(value: string, type: 'string'): Converter<string>
function generateConverter(value: string, type: 'number' | 'string'): Converter<number | string> {
return (value) => {
switch (type) {
case 'number':
return Number(value)
default:
return value
}
}
}
answered Nov 20 '18 at 15:03
Titian Cernicova-DragomirTitian Cernicova-Dragomir
60.9k33755
add a comment |
Typescript will not let you assign concrete values where a value of a generic type parameter is expected. The reason for this is that usually the value can't be validated against all possible derived types that could conceivable be passed for the generic type parameter. For example even in your case, T extends number| string so this mean it could be the number literal type 1 which would not be satisfied by the value Number(value).
In your case I would recommend doing away with the generic type parameters and use multiple overloads, with the implementation signature returning Converter<number | string>. This has the added benefit that the type of the converter will only need to be specified as a string not as both string and type parameter.
type Converter<T extends ConverterOutput> = (value: string) => T
type ConverterOutput = number | string
function generateConverter(value: string, type: 'number'): Converter<number>
function generateConverter(value: string, type: 'string'): Converter<string>
function generateConverter(value: string, type: 'number' | 'string'): Converter<number | string> {
return (value) => {
switch (type) {
case 'number':
return Number(value)
default:
return value
}
}
}
answered Nov 20 '18 at 15:03
Titian Cernicova-DragomirTitian Cernicova-Dragomir
60.9k33755
Typescript will not let you assign concrete values where a value of a generic type parameter is expected. The reason for this is that usually the value can't be validated against all possible derived types that could conceivable be passed for the generic type parameter. For example even in your case, T extends number| string so this mean it could be the number literal type 1 which would not be satisfied by the value Number(value).
In your case I would recommend doing away with the generic type parameters and use multiple overloads, with the implementation signature returning Converter<number | string>. This has the added benefit that the type of the converter will only need to be specified as a string not as both string and type parameter.
type Converter<T extends ConverterOutput> = (value: string) => T
type ConverterOutput = number | string
function generateConverter(value: string, type: 'number'): Converter<number>
function generateConverter(value: string, type: 'string'): Converter<string>
function generateConverter(value: string, type: 'number' | 'string'): Converter<number | string> {
return (value) => {
switch (type) {
case 'number':
return Number(value)
default:
return value
}
}
}
answered Nov 20 '18 at 15:03
Titian Cernicova-DragomirTitian Cernicova-Dragomir
60.9k33755
answered Nov 20 '18 at 15:03
Titian Cernicova-DragomirTitian Cernicova-Dragomir
60.9k33755
answered Nov 20 '18 at 15:03
Titian Cernicova-DragomirTitian Cernicova-Dragomir
60.9k33755
answered Nov 20 '18 at 15:03
Titian Cernicova-DragomirTitian Cernicova-Dragomir
60.9k33755
60.9k33755
add a comment |
add a comment |
Because a number nor a string does not extend itself. That's why it's not working. You can fix it by giving the type directly e.g.
function generateConverter(value: string, type: 'number' | 'string'): Converter<ConverterOutput> {
return (value: string): ConverterOutput => {
switch (type) {
case 'number':
return Number(value)
default:
return value
}
}
}
answered Nov 20 '18 at 15:03
Murat KaragözMurat Karagöz
14.5k53568
add a comment |
Because a number nor a string does not extend itself. That's why it's not working. You can fix it by giving the type directly e.g.
function generateConverter(value: string, type: 'number' | 'string'): Converter<ConverterOutput> {
return (value: string): ConverterOutput => {
switch (type) {
case 'number':
return Number(value)
default:
return value
}
}
}
answered Nov 20 '18 at 15:03
Murat KaragözMurat Karagöz
14.5k53568
add a comment |
Because a number nor a string does not extend itself. That's why it's not working. You can fix it by giving the type directly e.g.
function generateConverter(value: string, type: 'number' | 'string'): Converter<ConverterOutput> {
return (value: string): ConverterOutput => {
switch (type) {
case 'number':
return Number(value)
default:
return value
}
}
}
answered Nov 20 '18 at 15:03
Murat KaragözMurat Karagöz
14.5k53568
Because a number nor a string does not extend itself. That's why it's not working. You can fix it by giving the type directly e.g.
function generateConverter(value: string, type: 'number' | 'string'): Converter<ConverterOutput> {
return (value: string): ConverterOutput => {
switch (type) {
case 'number':
return Number(value)
default:
return value
}
}
}
answered Nov 20 '18 at 15:03
Murat KaragözMurat Karagöz
14.5k53568
answered Nov 20 '18 at 15:03
Murat KaragözMurat Karagöz
14.5k53568
answered Nov 20 '18 at 15:03
Murat KaragözMurat Karagöz
14.5k53568
answered Nov 20 '18 at 15:03
Murat KaragözMurat Karagöz
14.5k53568
14.5k53568
add a comment |
add a comment |
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