I learned a lot from the post, and this is why I registered account to write my first post.
https://medium.com/free-code-camp/typescript-curry-ramda-types-f747e99744ab
0x00 what we want to do
We want to do like this ↓↓↓ use TS Type solve FIbonacci
type r0 = Fib<Zero>;
// type r10= 0
type r1 = Fib<One>;
// type r1 = 1
type r2 = Fib<Two>;
// type r2 = 1
type r3 = Fib<3>;
// type r3 = 2
type r4 = Fib<4>;
// type r4 = 3
type r5 = Fib<5>;
// type r5 = 5
type r6 = Fib<6>;
// type r6 = 8
1x00 How we can do this
1x01 First, we need some util types
They are simple and very easy to understand
- Range: generate list
- Length: get list's size
- Concat: concat two list
type Length<T extends any[]> = T["length"];
type Range<T extends Number = 0, P extends any[] = []> = {
0: Range<T, [any, ...P]>;
1: P;
}[Length<P> extends T ? 1 : 0];
type Concat<T extends any[], P extends any[]> = [...T, ...P];
type t1 = Range<3>;
// type t1 = [any, any, any]
type Zero = Length<Range<0>>;
// type Zero = 0
type One = Length<Range<1>>;
// type One = 1
type Ten = Length<Range<10>>;
// type Ten = 10
type Five = Length<Range<5>>;
// type Five = 5
type Six = Length<Concat<Range<5>, Range<1>>>;
// type Six = 6
Add is also easy
- We generate two list
- Concat them
- Get the result size
type Add<T extends number, P extends number> = Length<
Concat<Range<T>, Range<P>>
>;
type Two = Add<One, One>;
// type Two = 2
type Three = Add<One, Two>;
// type Three = 3
But how to implement subtraction?
1x02 We need more util types
Some array types
- Append: insert element in head of list
- IsEmpty/NotEmpty: judge list is/not empty
- Tail: delete the first element
type Append<T extends any[], E = any> = [...T, E];
type IsEmpty<T extends any[]> = Length<T> extends 0 ? true : false;
type NotEmpty<T extends any[]> = IsEmpty<T> extends true ? false : true;
type t4 = IsEmpty<Range<0>>;
// type t4 = true
type t5 = IsEmpty<Range<1>>;
// type t5 = false
type Tail<T extends any[]> = ((...t: T) => any) extends (
_: any,
...tail: infer P
) => any
? P
: [];
type t22 = Tail<[1, 2, 3]>;
// type t22 = [2, 3]
type t23 = Tail<[1]>;
// type t23 = []
type t24 = Tail<[]>;
// type t24 = []
logic type
- And: a && b
- LessList: a.length <= b.length
- Less: a <= b
type And<T extends boolean, P extends boolean> = T extends false
? false
: P extends false
? false
: true;
type t6 = And<true, true>;
// type t6 = true
type t7 = And<true, false>;
// type t7 = false
type t8 = And<false, false>;
// type t8 = false
type t9 = And<false, true>;
// type t9 = false
// T <= P
type LessList<T extends any[], P extends any[]> = {
0: LessList<Tail<T>, Tail<P>>;
1: true;
2: false;
}[And<NotEmpty<T>, NotEmpty<P>> extends true
? 0
: IsEmpty<T> extends true
? 1
: 2];
type Less<T extends number, P extends number> = LessList<Range<T>, Range<P>>;
type t10 = Less<Zero, One>;
// type t10 = true
type t11 = Less<One, Zero>;
// type t11 = false
type t12 = Less<One, One>;
// type t12 = true
Now we can 'translate' js to ts
- SubList:
const a = [1, 2, 3];
const b = [4, 5];
const c = [];
while (b.length !== a.length) {
a.pop();
c.push(1);
}
// c.length === a.length - b.length
console.log(c.length);
- Sub: a - b
type SubList<T extends any[], P extends any[], R extends any[] = []> = {
0: Length<R>;
1: SubList<Tail<T>, P, Append<R>>;
}[Length<T> extends Length<P> ? 0 : 1];
type t13 = SubList<Range<10>, Range<5>>;
// type t13 = 5
// T - P
type Sub<T extends number, P extends number> = {
0: Sub<P, T>;
1: SubList<Range<T>, Range<P>>;
}[Less<T, P> extends true ? 0 : 1];
type t14 = Sub<One, Zero>;
// type t14 = 1
type t15 = Sub<Ten, Five>;
// type t15 = 5
2x00 JS Function ==> TS Type
In js we use function
const fib = (n) => (n <= 1 ? n : n++);
in ts we use type!!! they look like same!
type Fib<T extends number> = {
0: T;
1: Add<Fib<Sub<T, One>>, Fib<Sub<T, Two>>>;
}[Less<T, One> extends true ? 0 : 1];
type r0 = Fib<Zero>;
// type r10= 0
type r1 = Fib<One>;
// type r1 = 1
type r2 = Fib<Two>;
// type r2 = 1
type r3 = Fib<3>;
// type r3 = 2
type r4 = Fib<4>;
// type r4 = 3
type r5 = Fib<5>;
//type r5 = 5
type r6 = Fib<6>;
// type r6 = 8
Finally, we use ts solve Fibonacci, This is amazing! I'd never thought of doing that before. Thanks to those who share their genius thoughts 💖~
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