How to use function overloading to implement a bidirectional mapping function?

TL;DR

Try it online

I have two union types (both string literals) and a hard coded mapping object which maps these two types to each other.

type Digit = '1' | '2'

type Alpha = 'a' | 'b'

const map: Record<Digit, Alpha> & Record<Alpha, Digit> = {

    '1': 'a',

    '2': 'b',

    'a': '1',

    'b': '2'

}

The hard coded map works nice when being used agains singleton values.

let digit: Digit = '1'

let alpha = map[digit] // alpha is inferred as Alpha

let digit2 = map[alpha] // digit2 is inferred as Digit

Wrapping it with a function and then mapping over an array is also possible.

let digitArr: Digit = ['1', '2']

let alphaArr = digitArr.map(d => map[d]) // inferred as Alpha

But when I want to use only one function, with overloading, to achieve bidirectional mapping, i.e.

digitArr.map(mapping) // returns Alpha

alphaArr.map(mapping) // returns Digit

// where digitArr and alphaArr is deterministically typed as Digit and Alpha

the type system complains about it and I don't know how to make it happy.

My function definition:

function overloadedMapping(digit: Digit): Alpha

function overloadedMapping(alpha: Alpha): Digit

function overloadedMapping(key: Digit | Alpha): Digit | Alpha {

    return map[key]

}

This function also works nice with singleton values and fails on one kind of array.

let d = overloadedMapping('a') // d is of type Digit

let a = overloadedMapping('1') // a is of type Alpha

let arr = alphaArr.map(overloadedMapping) // arr is of type Digit. works? why?

let arr2 = digitArr.map(overloadedMapping) // type inference fails!! why?

An interesting observation: When the overloading declaration order is switched, the array mapping which fails also alternates.

Try it online(same as the first link)

asked Dec 28 '18 at 3:35

Nandiin Bao

927520

1

Just a side note: enums are bidirectional maps already built into TypeScript. You can create one (enum MyMap { Alpha = 0, Beta = 1}) and access both keys and values (MyMap['Alpha'], MyMap[0]).
– Karol Majewski
Dec 28 '18 at 3:50

Hi Karol, thanks. While enum does provide a bidirectional mapping, it doesn't match my needs. Both side of the mapping needs to be arbitrary string literals and a string based enum is not bidirectional mapped. And still, mapping over an array with one function is not possible.
– Nandiin Bao
Dec 28 '18 at 5:56

add a comment |

TL;DR

Try it online

I have two union types (both string literals) and a hard coded mapping object which maps these two types to each other.

type Digit = '1' | '2'

type Alpha = 'a' | 'b'

const map: Record<Digit, Alpha> & Record<Alpha, Digit> = {

    '1': 'a',

    '2': 'b',

    'a': '1',

    'b': '2'

}

The hard coded map works nice when being used agains singleton values.

let digit: Digit = '1'

let alpha = map[digit] // alpha is inferred as Alpha

let digit2 = map[alpha] // digit2 is inferred as Digit

Wrapping it with a function and then mapping over an array is also possible.

let digitArr: Digit = ['1', '2']

let alphaArr = digitArr.map(d => map[d]) // inferred as Alpha

But when I want to use only one function, with overloading, to achieve bidirectional mapping, i.e.

digitArr.map(mapping) // returns Alpha

alphaArr.map(mapping) // returns Digit

// where digitArr and alphaArr is deterministically typed as Digit and Alpha

the type system complains about it and I don't know how to make it happy.

My function definition:

function overloadedMapping(digit: Digit): Alpha

function overloadedMapping(alpha: Alpha): Digit

function overloadedMapping(key: Digit | Alpha): Digit | Alpha {

    return map[key]

}

This function also works nice with singleton values and fails on one kind of array.

let d = overloadedMapping('a') // d is of type Digit

let a = overloadedMapping('1') // a is of type Alpha

let arr = alphaArr.map(overloadedMapping) // arr is of type Digit. works? why?

let arr2 = digitArr.map(overloadedMapping) // type inference fails!! why?

An interesting observation: When the overloading declaration order is switched, the array mapping which fails also alternates.

Try it online(same as the first link)

asked Dec 28 '18 at 3:35

Nandiin Bao

927520

1

Just a side note: enums are bidirectional maps already built into TypeScript. You can create one (enum MyMap { Alpha = 0, Beta = 1}) and access both keys and values (MyMap['Alpha'], MyMap[0]).
– Karol Majewski
Dec 28 '18 at 3:50

Hi Karol, thanks. While enum does provide a bidirectional mapping, it doesn't match my needs. Both side of the mapping needs to be arbitrary string literals and a string based enum is not bidirectional mapped. And still, mapping over an array with one function is not possible.
– Nandiin Bao
Dec 28 '18 at 5:56

add a comment |

TL;DR

Try it online

I have two union types (both string literals) and a hard coded mapping object which maps these two types to each other.

type Digit = '1' | '2'

type Alpha = 'a' | 'b'

const map: Record<Digit, Alpha> & Record<Alpha, Digit> = {

    '1': 'a',

    '2': 'b',

    'a': '1',

    'b': '2'

}

The hard coded map works nice when being used agains singleton values.

let digit: Digit = '1'

let alpha = map[digit] // alpha is inferred as Alpha

let digit2 = map[alpha] // digit2 is inferred as Digit

Wrapping it with a function and then mapping over an array is also possible.

let digitArr: Digit = ['1', '2']

let alphaArr = digitArr.map(d => map[d]) // inferred as Alpha

But when I want to use only one function, with overloading, to achieve bidirectional mapping, i.e.

digitArr.map(mapping) // returns Alpha

alphaArr.map(mapping) // returns Digit

// where digitArr and alphaArr is deterministically typed as Digit and Alpha

the type system complains about it and I don't know how to make it happy.

My function definition:

function overloadedMapping(digit: Digit): Alpha

function overloadedMapping(alpha: Alpha): Digit

function overloadedMapping(key: Digit | Alpha): Digit | Alpha {

    return map[key]

}

This function also works nice with singleton values and fails on one kind of array.

let d = overloadedMapping('a') // d is of type Digit

let a = overloadedMapping('1') // a is of type Alpha

let arr = alphaArr.map(overloadedMapping) // arr is of type Digit. works? why?

let arr2 = digitArr.map(overloadedMapping) // type inference fails!! why?

An interesting observation: When the overloading declaration order is switched, the array mapping which fails also alternates.

Try it online(same as the first link)

asked Dec 28 '18 at 3:35

Nandiin Bao

927520

TL;DR

Try it online

I have two union types (both string literals) and a hard coded mapping object which maps these two types to each other.

type Digit = '1' | '2'

type Alpha = 'a' | 'b'

const map: Record<Digit, Alpha> & Record<Alpha, Digit> = {

    '1': 'a',

    '2': 'b',

    'a': '1',

    'b': '2'

}

The hard coded map works nice when being used agains singleton values.

let digit: Digit = '1'

let alpha = map[digit] // alpha is inferred as Alpha

let digit2 = map[alpha] // digit2 is inferred as Digit

Wrapping it with a function and then mapping over an array is also possible.

let digitArr: Digit = ['1', '2']

let alphaArr = digitArr.map(d => map[d]) // inferred as Alpha

But when I want to use only one function, with overloading, to achieve bidirectional mapping, i.e.

digitArr.map(mapping) // returns Alpha

alphaArr.map(mapping) // returns Digit

// where digitArr and alphaArr is deterministically typed as Digit and Alpha

the type system complains about it and I don't know how to make it happy.

My function definition:

function overloadedMapping(digit: Digit): Alpha

function overloadedMapping(alpha: Alpha): Digit

function overloadedMapping(key: Digit | Alpha): Digit | Alpha {

    return map[key]

}

This function also works nice with singleton values and fails on one kind of array.

let d = overloadedMapping('a') // d is of type Digit

let a = overloadedMapping('1') // a is of type Alpha

let arr = alphaArr.map(overloadedMapping) // arr is of type Digit. works? why?

let arr2 = digitArr.map(overloadedMapping) // type inference fails!! why?

An interesting observation: When the overloading declaration order is switched, the array mapping which fails also alternates.

Try it online(same as the first link)

typescript typescript-typings function-overloading

asked Dec 28 '18 at 3:35

Nandiin Bao

927520

asked Dec 28 '18 at 3:35

Nandiin Bao

927520

asked Dec 28 '18 at 3:35

Nandiin Bao

927520

asked Dec 28 '18 at 3:35

Nandiin Bao

927520

asked Dec 28 '18 at 3:35

Nandiin Bao

927520

1

Just a side note: enums are bidirectional maps already built into TypeScript. You can create one (enum MyMap { Alpha = 0, Beta = 1}) and access both keys and values (MyMap['Alpha'], MyMap[0]).
– Karol Majewski
Dec 28 '18 at 3:50

Hi Karol, thanks. While enum does provide a bidirectional mapping, it doesn't match my needs. Both side of the mapping needs to be arbitrary string literals and a string based enum is not bidirectional mapped. And still, mapping over an array with one function is not possible.
– Nandiin Bao
Dec 28 '18 at 5:56

add a comment |

1

Just a side note: enums are bidirectional maps already built into TypeScript. You can create one (enum MyMap { Alpha = 0, Beta = 1}) and access both keys and values (MyMap['Alpha'], MyMap[0]).
– Karol Majewski
Dec 28 '18 at 3:50

Hi Karol, thanks. While enum does provide a bidirectional mapping, it doesn't match my needs. Both side of the mapping needs to be arbitrary string literals and a string based enum is not bidirectional mapped. And still, mapping over an array with one function is not possible.
– Nandiin Bao
Dec 28 '18 at 5:56

Just a side note: enums are bidirectional maps already built into TypeScript. You can create one (enum MyMap { Alpha = 0, Beta = 1}) and access both keys and values (MyMap['Alpha'], MyMap[0]).
– Karol Majewski
Dec 28 '18 at 3:50

Hi Karol, thanks. While enum does provide a bidirectional mapping, it doesn't match my needs. Both side of the mapping needs to be arbitrary string literals and a string based enum is not bidirectional mapped. And still, mapping over an array with one function is not possible.
– Nandiin Bao
Dec 28 '18 at 5:56

add a comment |

1 Answer
1

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When overload resolution fails to match your callback with the desired signature, you can tell TypeScript what your intention is by providing the type parameter explicitly:

let arr2 = digitArr.map<Alpha>(overloadedMapping)

Now your transformation will be treated as (value: Digit, index: number, array: Digit) => Alpha.

In this case, opting out of the point-free style will help TypeScript match the overload as well:

let arr2 = digitArr.map(digit => overloadedMapping(digit))

answered Dec 28 '18 at 14:34

Karol Majewski

1,22617

add a comment |

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1 Answer
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1 Answer
1

active

oldest

votes

When overload resolution fails to match your callback with the desired signature, you can tell TypeScript what your intention is by providing the type parameter explicitly:

let arr2 = digitArr.map<Alpha>(overloadedMapping)

Now your transformation will be treated as (value: Digit, index: number, array: Digit) => Alpha.

In this case, opting out of the point-free style will help TypeScript match the overload as well:

let arr2 = digitArr.map(digit => overloadedMapping(digit))

answered Dec 28 '18 at 14:34

Karol Majewski

1,22617

add a comment |

When overload resolution fails to match your callback with the desired signature, you can tell TypeScript what your intention is by providing the type parameter explicitly:

let arr2 = digitArr.map<Alpha>(overloadedMapping)

Now your transformation will be treated as (value: Digit, index: number, array: Digit) => Alpha.

In this case, opting out of the point-free style will help TypeScript match the overload as well:

let arr2 = digitArr.map(digit => overloadedMapping(digit))

answered Dec 28 '18 at 14:34

Karol Majewski

1,22617

add a comment |

When overload resolution fails to match your callback with the desired signature, you can tell TypeScript what your intention is by providing the type parameter explicitly:

let arr2 = digitArr.map<Alpha>(overloadedMapping)

Now your transformation will be treated as (value: Digit, index: number, array: Digit) => Alpha.

In this case, opting out of the point-free style will help TypeScript match the overload as well:

let arr2 = digitArr.map(digit => overloadedMapping(digit))

answered Dec 28 '18 at 14:34

Karol Majewski

1,22617

When overload resolution fails to match your callback with the desired signature, you can tell TypeScript what your intention is by providing the type parameter explicitly:

let arr2 = digitArr.map<Alpha>(overloadedMapping)

Now your transformation will be treated as (value: Digit, index: number, array: Digit) => Alpha.

In this case, opting out of the point-free style will help TypeScript match the overload as well:

let arr2 = digitArr.map(digit => overloadedMapping(digit))

answered Dec 28 '18 at 14:34

Karol Majewski

1,22617

answered Dec 28 '18 at 14:34

Karol Majewski

1,22617

answered Dec 28 '18 at 14:34

Karol Majewski

1,22617

answered Dec 28 '18 at 14:34

Karol Majewski

1,22617

add a comment |

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