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meta-types

TypeScript meta functions for (especially variadic) meta programming

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Arithmetics

The arithmetics in this library handles positive integers up to ~98, sometimes more.

Add

Add two positive numbers:

import type { Add } from 'meta-types'
type T = Add< 13, 11 >; // T is 24

Sub

Subtract the second number from the first number:

import type { Sub } from 'meta-types'
type T = Sub< 13, 11 >; // T is 2

Mul

Multiply two numbers:

import type { Mul } from 'meta-types'
type T = Mul< 13, 11 >; // T is 143

Conditionals and comparisons

Conditional

Return Then if If otherwise Else, or; if If then Then else Else.

import type { If, Extends } from 'meta-types'
type T1 = If< true, "yes", "no" >;  // T1 is "yes"
type T2 = If< false, "yes", "no" >; // T2 is "no"
type T3 = If< Extends< "42", number >, "yes", "no" >; // T3 is "no", "42" doesn't extend number

Extends

Returns true if T extends E, otherwise false. This is a trivial operation, but practical in meta functional statements, e.g. If.

import type { Extends } from 'meta-types'
type T1 = Extends< true, boolean >; // T1 is true; true extends boolean
type T1 = Extends< "42", number >;  // T2 is false; "42" doesn't extend number

GreaterThan

GreaterThan returns true if the first number is greater than the second.

A third argument can be set to true, to turn this into greater-than-or-equal.

import type { GreaterThan } from 'meta-types'
type T1 = GreaterThan< 42, 40 >;       // T1 is true; 42 > 40
type T2 = GreaterThan< 40, 40, true >; // T2 is true; 40 >= 40
type T3 = GreaterThan< 40, 42 >;       // T3 is false; 40 < 42

Is

Is returns true for equal types.

import type { Is } from 'meta-types'
type T1 = Is< 42, number >;      // T1 is false
type T2 = Is< null, undefined >; // T2 is false
type T3 = Is< "foo", "foo" >;    // T3 is true

IsFalsy

IsFalsy returns true for any falsy type (false, "", 0, null and undefined).

import type { IsFalsy } from 'meta-types'
type T1 = IsFalsy< 0 >; // T1 is true
type T2 = IsFalsy< 3 >; // T2 is false

Logics

Logic (and, or, xor)

And, Or and Xor can be used to perform logical operations on booleans.

import type { And, Or, Xor } from 'meta-types'
type T1 = And< true, false >; // T1 is false, {true && false} -> false
type T2 = Or<  true, false >; // T2 is true, {true || false} -> true
type T2 = Xor< true, false >; // T3 is true, {true ^ false} -> true

Not

Invert true or false using Not.

import type { Not } from 'meta-types'
type F = Not< true >;  // F is false, !true
type T = Not< false >; // T is true, !false

A second argument Strict can be provided, which defaults to true. If this is set to false, and falsy type will return true instead of never.

import type { Not } from 'meta-types'
type F = Not< 0 >;        // F is never, not exactly false
type T = Not< 0, false >; // T is true, 0 is falsy

Tuples

Fill

Create a tuple of size N, filled with any (or type T if provided).

import type { Fill } from 'meta-types'
type T1 = Fill< 3 >; // T1 is [ any, any, any ]
type T2 = Fill< 4, null >; // T2 is [ null, null, null, null ]

First, Last

First and Last can be used to extract the first and last elements in a tuple.

import type { First, Last } from 'meta-types'
type T = [ "hello", 42, "world", true ];
type T1 = First< T >; // T1 is "hello"
type T2 = Last< T >;  // T2 is true

Rest, ReverseRest

Rest and ReverseRest can be used to extract all-but-the-first or all-but-the-last elements in a tuple.

import type { Rest, ReverseRest } from 'meta-types'
type T = [ "hello", 42, "world", true ];
type T1 = Rest< T >;         // T1 is [ 42, "world", true ]
type T2 = ReverseRest< T >;  // T2 is [ "hello", 42, "world" ]

LargerThan, SameLength

LargerThan and SameLength can be used to extract and compare the length of tuples. A second argument can be provided to LargerThan to turn it into larger-than-or-same-length.

import type { LargerThan, SameLength } from 'meta-types'
type T1 = LargerThan< [ 0, 0 ], [ 0, 0, 0 ] >;    // T1 is false, first length 2 < second length 3
type T2 = SameLength< [ 0, 0 ], [ 0, 0 ] >;       // T2 is true, length 2 === length 2
type T3 = LargerThan< [ 0, 0 ], [ 0, 0 ], true >; // T3 is true, 2 >= 2

LengthOf

LengthOf returns the length of a tuple, or a fallback type (defaults to -1) if the tuple is unbounded, i.e. it's rest-spread like [ "foo", 42, ...null[] ].

import type { LengthOf } from 'meta-types'
type T1 = LengthOf< [ 0, 0 ] >;                 // T1 is 2
type T2 = LengthOf< [ 0, 0, ...any ] >;         // T2 is -1
type T3 = LengthOf< [ 0, 0, ...any ], "ouch" >; // T3 is "ouch"

Optional

Append an optional tuple to a required tuple.

import type { Optional } from 'meta-types'
type T = Optional< [ string, number ], [ boolean, any ] >;
// T is:
//   | [ string, number ]
//   | [ string, number, boolean ]
//   | [ string, number, boolean, any ]

Turn all types after index N into optional.

import type { OptionalFrom } from 'meta-types'
type T = OptionalFrom< [ string, number, boolean, any ], 2 >;
// T is:
//   | [ string, number ]
//   | [ string, number, boolean ]
//   | [ string, number, boolean, any ]

Overwrite

Overwrite the first elements in a tuple, with elements from another tuple. The first tuple will be overwritten with types in the second tuple.

import type { Overwrite } from 'meta-types'
type T1 = Overwrite< [ 1, 2, 3 ], [ "a", "b" ] >; // T1 is [ "a", "b", 3 ]
type T2 = Overwrite< [ 1, 2 ], [ "a", "b", "c" ] >; // T2 is [ "a", "b", "c" ]

Reverse

Reverse a tuple.

import type { Reverse } from 'meta-types'
type T = Reverse< [ string, boolean, 42 ] >; // T is [ 42, boolean, string ]

Shift

Shift (remove the first) N elements from a tuple.

import type { Shift } from 'meta-types'
type T = Shift< [ string, boolean, number, 42, "foo" ], 2 >;
// T is [ number, 42, "foo" ]

Slice

Slice a tuple with a start and end offset.

import type { Slice } from 'meta-types'
type T = Slice< [ string, boolean, number, 42, "foo" ], 2, 4 >;
// T is [ number, 42 ]

Truncate

Truncate a tuple to a max length.

import type { Truncate } from 'meta-types'
type T = Truncate< [ string, boolean, number, 42, "foo" ], 2 >;
// T is [ string, boolean ]

Union

Union all the types in a tuple, with a type.

import type { UnionAll } from 'meta-types'
type T = UnionAll< [ string, boolean, number ], U >;
// T is [ string | U, boolean | U, number | U ]

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