README

type-unlambda - Unlambda interpreter implemented in TypeScript's type system

Getting Started

Installation:

npm install --save-dev typescript
npm install --save-dev @esolangs/type-unlambda

Example usage:

import Unlambda from '@esolangs/type-unlambda';

type Code = '``@c`d``s`|k`@c';
type Input = 'Hello!';
type Output = Unlambda<Code, Input>;  // Output == '!olleH'

Screenshots:

Notes

You're likely to get the following error when trying to run a program with type-unlambda:

Type instantiation is excessively deep and possibly infinite.ts(2589).

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 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.

In src/compiler/checker.ts, there is a hard-coded limit for type instantiation:

if (instantiationDepth === 50 || instantiationCount >= 5000000) {
    // ...
    return errorType;
}

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.

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.