From cca866ae624791e6ac7460670e68b535639c9391 Mon Sep 17 00:00:00 2001
From: Bram van den Heuvel <bram.vdh1999@gmail.com>
Date: Mon, 29 Jun 2026 11:59:35 +0200
Subject: [PATCH] Add minimal proof checker

---
 proof.py | 232 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 232 insertions(+)
 create mode 100644 proof.py

diff --git a/proof.py b/proof.py
new file mode 100644
index 0000000..c3046dd
--- /dev/null
+++ b/proof.py
@@ -0,0 +1,232 @@
+from __future__ import annotations
+from typing import Any, Callable, Union
+
+from dataclasses import dataclass
+
+# -----------------------------------------------------------------------------
+# Terms
+# -----------------------------------------------------------------------------
+
+class Term:
+    pass
+
+@dataclass(frozen=True)
+class App(Term):
+    f : Term
+    x : Term
+
+@dataclass(frozen=True)
+class Const(Term):
+    name : str
+
+@dataclass(frozen=True)
+class Var(Term):
+    name : str
+
+# --- Bool constructors ---
+
+Bool = Union["Truth", "Contradiction"]
+
+@dataclass(frozen=True)
+class Truth(Term):
+    pass
+
+@dataclass(frozen=True)
+class Contradiction(Term):
+    pass
+
+# --- Nat constructors ---
+
+Nat = Union["Zero", "Succ"]
+
+@dataclass(frozen=True)
+class Zero(Term):
+    pass
+
+@dataclass(frozen=True)
+class Succ(Term):
+    n : Term
+
+def kernel_from_int(n : int) -> Nat:
+    if n == 0:
+        return Zero()
+    elif n < 0:
+        raise ValueError("Int is not Nat")
+    else:
+        return Succ(n=kernel_from_int(n=n-1))
+
+# -----------------------------------------------------------------------------
+# Propositions
+# -----------------------------------------------------------------------------
+
+class Prop:
+    pass
+
+@dataclass(frozen=True)
+class Eq(Prop):
+    lhs : Term
+    rhs : Term
+
+# -----------------------------------------------------------------------------
+# Proofs
+# -----------------------------------------------------------------------------
+
+class Proof:
+    pass
+
+@dataclass(frozen=True)
+class Refl(Proof):
+    pass
+
+# -----------------------------------------------------------------------------
+# Normalization function
+# -----------------------------------------------------------------------------
+
+@dataclass(frozen=True)
+class F:
+    arity : int
+    func : Callable[..., Term | None]
+
+def apply_rules() -> dict[str, F]:
+    def __bool_not(x : Term) -> Term | None:
+        match x:
+            case Truth():
+                return Contradiction()
+            
+            case Contradiction():
+                return Truth()
+    
+    def __nat_add(x : Term, y : Term) -> Term | None:
+        match x:
+            case Zero():
+                return y
+            
+            case Succ():
+                return Succ(
+                    n=normalize(App(
+                        f=App(Const("Nat.add"), x=x.n),
+                        x=y,
+                    ))
+                )
+    
+    def __nat_iszero(x : Term) -> Term | None:
+        match x:
+            case Zero():
+                return Truth()
+            
+            case Succ():
+                return Contradiction()
+    
+    return {
+        "Bool.not": F(arity=1, func=__bool_not),
+        "Nat.add" : F(arity=2, func=__nat_add),
+        "Nat.isZero" : F(arity=1, func=__nat_iszero),
+    }
+                
+
+def normalize(term : Term) -> Term:
+    match term:
+        case App():
+            f = normalize(term.f)
+            x = normalize(term.x)
+
+            cursor = term
+            items = [ term ]
+
+            while True:
+                match cursor.f:
+                    case App():
+                        cursor = cursor.f
+                        items.append(cursor)
+                    
+                    case Const():
+                        name = cursor.f.name
+                        rule = apply_rules().get(name, None)
+
+                        if rule is None:
+                            return App(f, x) # Unknown function
+                        if len(items) != rule.arity:
+                            # If len(items) is too small, we don't have
+                            # enough inputs yet to evaluate the function
+                            # If len(items) is too large, we already evaluated
+                            # the function before and couldn't optimize.
+                            return App(f, x)
+                        
+                        out = rule.func(*reversed([i.x for i in items]))
+
+                        return out if out is not None else App(f, x)
+
+                    case Contradiction():
+                        if len(items) != 2:
+                            # Either evaluating too early or too late
+                            return term
+                        
+                        # Return the "second" item
+                        return x
+
+                    case Truth():
+                        if len(items) != 2:
+                            # Either evaluating too early or too late
+                            return App(f, x)
+                        
+                        # Return the "first" item
+                        return cursor.x
+
+        case Const():
+            return term
+        
+        case Succ():
+            return Succ(n=normalize(term.n))
+        
+        case Term():
+            return term
+
+        case Var():
+            return term
+        
+        case Zero():
+            return term
+
+# -----------------------------------------------------------------------------
+# Proof checker
+# -----------------------------------------------------------------------------
+
+def check(goal: Prop, proof : Proof) -> bool:
+    match ( goal, proof ):
+        case ( Eq(), Refl() ):
+            return normalize(goal.lhs) == normalize(goal.rhs)
+    
+        case _:
+            return False
+
+# -----------------------------------------------------------------------------
+# -----------------------------------------------------------------------------
+# -----------------------------------------------------------------------------
+
+if __name__ == "__main__":
+    # 2 + 3 == 5
+    print(check(
+        Eq(
+            lhs=App(f=App(f=Const("Nat.add"), x=kernel_from_int(2)), x=kernel_from_int(3)),
+            rhs=kernel_from_int(5),
+        ),
+        Refl(),
+    ))
+
+    # 0 + x == x
+    print(check(
+        Eq(
+            lhs=App(f=App(f=Const("Nat.add"), x=Zero()), x=Var("x")),
+            rhs=Var("x")
+        ),
+        Refl(),
+    ))
+
+    # x + 0 == x
+    print(check(
+        Eq(
+            lhs=App(f=App(f=Const("Nat.add"), x=Var("x")), x=Zero()),
+            rhs=Var("x")
+        ),
+        Refl(),
+    ))