50 lines
1.9 KiB
Markdown
50 lines
1.9 KiB
Markdown
# README
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[](LICENSE)
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type-unlambda - [Unlambda](http://www.madore.org/~david/programs/unlambda) interpreter implemented in TypeScript's type system
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## Getting Started
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Installation:
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```sh
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npm install --save-dev typescript
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npm install --save-dev @esolangs/type-unlambda
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```
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Example usage:
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```typescript
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import Unlambda from '@esolangs/type-unlambda';
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type Code = '``@c`d``s`|k`@c';
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type Input = 'Hello!';
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type Output = Unlambda<Code, Input>; // Output == '!olleH'
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```
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Screenshots:
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## Notes
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You're likely to get the following error when trying to run a program with type-unlambda:
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> Type instantiation is excessively deep and possibly infinite.ts(2589).
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To write loops in TypeScript's type system, we have to use recursions, like we do in other purely functional programming languages. Meanwhile, we use [CPS](https://en.wikipedia.org/wiki/Continuation-passing_style) to implement continuations, which also introduces heavy recursion. However, TypeScript's type system is not meant for general purpose programming, and recursion has its limits.
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In [src/compiler/checker.ts](https://raw.githubusercontent.com/microsoft/TypeScript/release-4.1/src/compiler/checker.ts), there is a hard-coded limit for type instantiation:
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```typescript
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if (instantiationDepth === 50 || instantiationCount >= 5000000) {
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// ...
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return errorType;
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}
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```
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You may expect that there is an option somewhere that this limit can be configured, like `-ftemplate-depth=n` in gcc/clang. Unfortunately, there isn't, [and it's likely to stay that way](https://github.com/microsoft/TypeScript/pull/29602).
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To workaround this limitation, we modify the code of `tsserver` or `tsc` in `node_modules` until the error no longer applies. Changing `instantiationDepth` to `1000` is sufficient to run the example above.
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