branching and very basic merging of slots

versioning/value.js

@@ -0,0 +1,95 @@
+import { deepEqual } from "../util/util.js";
+import {inspect} from "node:util";
+
+// A Value is either:
+//   - a literal, without any dependencies.
+//   - read from a slot. the Value then has a read-dependency on that slot.
+//   - a transformation of another Value, by a function. the function is also a Value.
+
+// a -> Value<a>
+export const newLiteral = val => ({
+  kind: "literal",
+  out: val,
+  [inspect.custom]: (depth, options, inspect) => `newLiteral(${inspect(val)})`,
+});
+
+// Slot<a> -> Value<a>
+export const read = slot => ({
+  kind: "read",
+  slot,
+  out: slot.value.out,
+  [inspect.custom]: (depth, options, inspect) => `read(${inspect(slot)})`,
+});
+
+// Value<a> -> Value<a -> b> -> Value<b>
+export const transform = input => fn => {
+  const output = fn.out(input.out);
+  const _inspect = (depth, options, inspect) => `transform(${inspect(input)}, ${inspect(fn)})`;
+  if (input.kind === "literal") {
+    // optimization: sandwich everything together
+    return {
+      kind: "literal",
+      out: output,
+      // [inspect.custom]: _inspect,
+    };
+  }
+  else {
+    return {
+      kind: "transformation",
+      in: input,
+      fn,
+      out: output,
+      // [inspect.custom]: _inspect,
+    };
+  }
+};
+
+// Value<a> -> Set<Slot<Any>>
+export const getReadDependencies = value => {
+  if (value.kind === "literal") {
+    return new Set();
+  }
+  else if (value.kind === "read") {
+    return new Set([value.slot]);
+  }
+  else if (value.kind === "transformation") {
+    return new Set([
+      ...getReadDependencies(value.in),
+      ...getReadDependencies(value.fn),
+    ]);
+  }
+};
+
+// for debugging
+export const verifyValue = (value, indent = 0) => {
+  let success = true;
+  const printIndent = (...args) => {
+    console.log(" ".repeat(indent * 2), ...args);
+  };
+  const compare = (a, b, kind) => {
+    if (typeof a === 'function' && typeof b === 'function') {
+      printIndent("note: cannot compare functions", `(${kind})`);
+    }
+    else if (deepEqual(a, b)) {
+      printIndent(`ok (${kind})`);
+    }
+    else {
+      printIndent(`bad (${kind})`);
+      success = false;
+    }
+  };
+  if (value.kind === "literal") {
+    compare(1, 1, "literal");
+  }
+  else if (value.kind === "read") {
+    compare(value.out, value.slot.value.out, "read");
+  }
+  else if (value.kind === "transformation") {
+    compare(value.fn.out(value.in.out),
+      value.out, "transformation");
+
+    success &= verifyValue(value.in, indent + 1);
+    success &= verifyValue(value.fn, indent + 1);
+  }
+  return success;
+};