From 4a0fb237cac8796e45d7b217db0d808452454820 Mon Sep 17 00:00:00 2001
From: Akshay Nair <phenax5@gmail.com>
Date: Sat, 16 Dec 2023 17:31:12 +0530
Subject: feat: lambda calculus

---
 src/lambda-calculus.ts | 114 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 114 insertions(+)
 create mode 100644 src/lambda-calculus.ts

(limited to 'src/lambda-calculus.ts')

diff --git a/src/lambda-calculus.ts b/src/lambda-calculus.ts
new file mode 100644
index 0000000..bbd2d1e
--- /dev/null
+++ b/src/lambda-calculus.ts
@@ -0,0 +1,114 @@
+import { Op, Rewrite, ChainRewrites, Sym, Equation, Refl, VerifyEquation, assert, Subst, ApplyRewrite, Evaluate, OpToStr, SubstEq } from './utils/theorem';
+
+export type Abs<x extends Op, o extends Op> = { op: '->', a: x, b: o }; // \x. o
+export type Ap<F extends Op, x extends Op> = { op: '$', a: F, b: x }; // F(x)
+export type Comp<F extends Op, G extends Op> = { op: '.', a: F, b: G }; // F . G
+export type Id<x extends Op = 'x'> = Abs<x, x> // \x. x
+export type Const<x extends Op = 'x', y extends Op = 'y'> = Abs<x, Abs<y, x>> // \x. \y. x
+export type Inv<F extends Op> = { op: 'inv', a: F }
+
+export namespace composition {
+  export type Abstraction<x extends Op, y extends Op, v extends Op> = Rewrite<Ap<Abs<x, y>, v>, y>;
+
+  export type Composition<F extends Op, G extends Op, x extends Op> = Rewrite<Ap<Comp<F, G>, x>, Ap<F, Ap<G, x>>>;
+
+  // Identity
+  export type Identity<x extends Op> = Rewrite<Ap<Id, x>, x>;
+  export namespace proof {
+    export interface Identity_Proof<x extends Op> {
+      type: 'rewrite';
+      left: Equation<Ap<Id<x>, x>, x>;  // id(x) = x
+      right: ChainRewrites<[
+        Abstraction<x, x, x>,  // x = x
+        Refl,
+      ], this['left']>;
+    }
+
+    export interface Identity_Composition_Proof<F extends Op, x extends Op> {
+      type: 'rewrite';
+      left: Equation<Ap<Comp<F, Id>, x>, Ap<F, x>>;  // (f . id)(x) = f(x)
+      right: ChainRewrites<[
+        Composition<F, Id, x>,    // f(id(x)) = f(x)
+        Identity<x>,              // f(x) = f(x)
+        Refl,
+      ], this['left']>;
+    }
+
+    export type identity = [
+      assert<VerifyEquation<Identity_Proof<'x'>>>,
+      assert<VerifyEquation<Identity_Composition_Proof<'F', 'x'>>>,
+    ]
+  }
+
+  // Associativity
+  export type Associativity<F extends Op, G extends Op, H extends Op> = Rewrite<
+    Comp<Comp<F, G>, H>,
+    Comp<F, Comp<G, H>>
+  >
+  export namespace proof {
+    export interface Assoc_Proof<F extends Op, G extends Op, H extends Op, x extends Op> {
+      type: 'rewrite';
+      left: Equation<Ap<Comp<Comp<F, G>, H>, x>, Ap<Comp<F, Comp<G, H>>, x>>;  // ((f . g) . h)(x) = (f . (g . h))(x)
+      right: ChainRewrites<[
+        Composition<Comp<F, G>, H, x>,       // (f . g)(h(x)) = (f . (g . h))(x)
+        Composition<F, G, Ap<H, x>>,         // f(g(h(x))) = (f . (g . h))(x)
+        Composition<F, Comp<G, H>, x>,       // f(g(h(x))) = f((g . h)(x))
+        Composition<G, H, x>,                // f(g(h(x))) = f(g(h(x)))
+        Refl,
+      ], this['left']>;
+    }
+    export type associativity = [
+      assert<VerifyEquation<Assoc_Proof<'F', 'G', 'H', 'x'>>>,
+    ]
+  }
+
+  // Identity
+  export type Transitivity<x extends Op, y extends Op, z extends Op> = Rewrite<Comp<Abs<y, z>, Abs<x, y>>, Abs<x, z>>;
+  export namespace proof {
+    export interface Transitivity_Proof<x extends Op, y extends Op, z extends Op> {
+      type: 'rewrite';
+      left: Equation<Ap<Comp<Abs<y, z>, Abs<x, y>>, x>, Ap<Abs<x, z>, x>>;  // ((\y -> z) . (\x -> y))(x) = (\x -> z)(x)
+      right: ChainRewrites<[
+        Composition<Abs<y, z>, Abs<x, y>, x>,        // (\y -> z) $ (\x -> y)(x) = (\x -> z)(x)
+        Abstraction<x, y, x>,                        // (\y -> z) $ y = (\x -> z)(x)
+        Abstraction<y, z, y>,                        // z = (\x -> z)(x)
+        Abstraction<x, z, x>,                        // z = z
+        Refl,
+      ], this['left']>;
+    }
+    export type transitivity = [
+      // OpToStr<Evaluate<Transitivity_Proof<'x', 'y', 'z'>>>,
+      assert<VerifyEquation<Transitivity_Proof<'x', 'y', 'z'>>>,
+    ]
+  }
+
+  // Inverse functions
+  export type Inverse<x extends Op, y extends Op> = Rewrite<Inv<Abs<x, y>>, Abs<y, x>>;
+  export namespace proof {
+    export interface Composition_Inverse_Proof<F extends Op, G extends Op> {
+      type: 'rewrite',
+      known: {
+        'f': Equation<F, Abs<'y', 'z'>>,
+        'g': Equation<G, Abs<'x', 'y'>>,
+      },
+      left: Equation<Inv<Comp<F, G>>, Comp<Inv<G>, Inv<F>>>, // inv(f . g) = inv(g) . inv(f)
+      right: ChainRewrites<[
+        SubstEq<this['known']['f']>,
+        SubstEq<this['known']['f']>,         // inv((\y -> z) . g) = inv(g) . inv(\y -> z)
+        SubstEq<this['known']['g']>,
+        SubstEq<this['known']['g']>,         // inv((\y -> z) . (\x -> y)) = inv(\x -> y) . inv(\y -> z)
+        Inverse<'x', 'y'>,
+        Inverse<'y', 'z'>,                   // inv((\x -> y) . (\y -> z)) = (\z -> y) . (\y -> x)
+        Transitivity<'x', 'y', 'z'>,         // inv(\x -> z) = (\z -> y) . (\y -> x)
+        Transitivity<'z', 'y', 'x'>,         // inv(\x -> z) = \z -> x
+        Inverse<'x', 'z'>,                   // \z -> x = \z -> x
+        Refl
+      ], this['left']>,
+    }
+    export type inverse = [
+      // OpToStr<Evaluate<Composition_Inverse_Proof<'F', 'G'>>>,
+      assert<VerifyEquation<Composition_Inverse_Proof<'F', 'G'>>>,
+    ]
+  }
+}
+
-- 
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