functionalscript · sergey-shandar · Aug 8, 2026 · Aug 7, 2026 · Aug 7, 2026 · Aug 7, 2026
diff --git a/fjs/asn.1/module.f.ts b/fjs/asn.1/module.f.ts
@@ -17,7 +17,7 @@ import {
     type Unpacked,
     type Vec
 } from "../types/bit_vec/module.f.ts"
-import { identity } from "../types/function/module.f.ts"
+import { identity } from "../types/function/module.f.mjs"
 import { max } from "../types/function/compare/module.f.ts"
 import { encode as b128encode, decode as b128decode } from "../basen/base128/module.f.ts"
 

diff --git a/fjs/base_n/module.f.ts b/fjs/base_n/module.f.ts
@@ -13,8 +13,8 @@
  */
 import { msb, lsb, type Vec, vec, chunkList, unpack } from '../types/bit_vec/module.f.ts'
 import { fold, type List } from '../types/list/module.f.ts'
-import { compose } from '../types/function/module.f.ts'
-import type { Nullable } from '../types/nullable/module.f.ts'
+import { compose } from '../types/function/module.f.mjs'
+import type { Nullable } from '../types/nullable/module.f.mjs'
 
 const { unpackSplit } = msb
 

diff --git a/fjs/basen/base64/module.f.ts b/fjs/basen/base64/module.f.ts
@@ -4,7 +4,7 @@
  * @module
  */
 import { msb, type Vec, length, vec, maxLength } from "../../types/bit_vec/module.f.ts"
-import type { Nullable } from "../../types/nullable/module.f.ts"
+import type { Nullable } from "../../types/nullable/module.f.mjs"
 import { baseN } from "../../base_n/module.f.ts"
 
 const alphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'

diff --git a/fjs/basen/cbase32/module.f.ts b/fjs/basen/cbase32/module.f.ts
@@ -4,7 +4,7 @@
  * @module
  */
 import { msb, lsb, type Vec, length, vec, empty } from "../../types/bit_vec/module.f.ts"
-import type { Nullable } from "../../types/nullable/module.f.ts"
+import type { Nullable } from "../../types/nullable/module.f.mjs"
 import { baseN } from "../../base_n/module.f.ts"
 
 //                         0123456789abcdef

diff --git a/fjs/bnf/data/proof.f.ts b/fjs/bnf/data/proof.f.ts
@@ -1,5 +1,5 @@
 import { stringify } from '../../media/json/module.f.ts'
-import { identity } from '../../types/function/module.f.ts'
+import { identity } from '../../types/function/module.f.mjs'
 import { sort } from '../../types/object/module.f.ts'
 import { oneEncode, option, range, rangeDecode, repeat0Plus, set } from '../module.f.ts'
 import { classic, deterministic } from '../testlib.f.ts'

diff --git a/fjs/bnf/token_symbol/module.f.ts b/fjs/bnf/token_symbol/module.f.ts
@@ -9,7 +9,7 @@
  * @module
  */
 import { assert } from '../../asserts/module.f.mjs'
-import { fromUndefined, type Nullable } from '../../types/nullable/module.f.ts'
+import { fromUndefined, type Nullable } from '../../types/nullable/module.f.mjs'
 import { eof, rangeDecode, unicodeRange } from '../module.f.ts'
 
 const [, unicodeLast] = rangeDecode(unicodeRange)

diff --git a/fjs/cas/evo/module.f.ts b/fjs/cas/evo/module.f.ts
@@ -53,7 +53,7 @@ import { decodeText, encodeText, dialect, checkReferences, isHash, type LockMap,
 import { ok, error, type Ok, type Result } from '../../types/result/module.f.ts'
 import { nonEmpty, empty as elEmpty } from '../../effects/list/module.f.ts'
 import { at, definedEntries, type StringMap } from '../../types/object/module.f.ts'
-import { unwrap } from '../../types/nullable/module.f.ts'
+import { unwrap } from '../../types/nullable/module.f.mjs'
 import type { Vec } from '../../types/bit_vec/module.f.ts'
 import { isNotFound, type IoResult } from '../../effects/node/module.f.ts'
 

diff --git a/fjs/cas/evo/proof.f.ts b/fjs/cas/evo/proof.f.ts
@@ -5,7 +5,7 @@ import { sha256 } from '../../crypto/sha2/module.f.ts'
 import { emptyState, virtual } from '../../effects/node/virtual/module.f.ts'
 import { vec, vec8, type Vec } from '../../types/bit_vec/module.f.ts'
 import { cBase32ToVec, vecToCBase32 } from '../../basen/cbase32/module.f.ts'
-import { unwrap } from '../../types/nullable/module.f.ts'
+import { unwrap } from '../../types/nullable/module.f.mjs'
 import { ok, error, type Ok } from '../../types/result/module.f.ts'
 import { nonEmpty, empty as elEmpty } from '../../effects/list/module.f.ts'
 import type { IoResult } from '../../effects/node/module.f.ts'

diff --git a/fjs/cas/module.f.ts b/fjs/cas/module.f.ts
@@ -35,7 +35,7 @@ import {
     type Stat,
     type WriteBytes,
 } from '../effects/node/module.f.ts'
-import { toOption } from '../types/nullable/module.f.ts'
+import { toOption } from '../types/nullable/module.f.mjs'
 import { error, ok, unwrap } from '../types/result/module.f.ts'
 import { splitAt } from '../types/string/module.f.ts'
 import { nonEmpty, empty as elEmpty, type List } from '../effects/list/module.f.ts'

diff --git a/fjs/ci/nix/module.f.ts b/fjs/ci/nix/module.f.ts
@@ -12,7 +12,7 @@
 import { forEachStep, mapStep, pure, step, type Effect } from '../../effects/module.f.ts'
 import { mkdir, writeUtf8File, type Mkdir, type WriteFile } from '../../effects/node/module.f.ts'
 import { nixToString, type Expression } from '../../media/nix/module.f.ts'
-import { fromUndefined, unwrap as unwrapNullable } from '../../types/nullable/module.f.ts'
+import { fromUndefined, unwrap as unwrapNullable } from '../../types/nullable/module.f.mjs'
 import { unwrap } from '../../types/result/module.f.ts'
 import { install, test, uses, type MetaStep } from '../common/module.f.ts'
 import { nixpkgs } from '../config/module.f.ts'

diff --git a/fjs/common/monoid/module.f.ts b/fjs/common/monoid/module.f.ts
@@ -8,7 +8,7 @@
  */
 import type { Fold, Reduce } from '../../types/function/operator/module.f.ts'
 import { reduce, type List } from '../../types/list/module.f.ts'
-import { flip } from '../../types/function/module.f.ts'
+import { flip } from '../../types/function/module.f.mjs'
 
 /**
  * Represents a monoid, an algebraic structure with a binary operation

diff --git a/fjs/crypto/pow/module.f.ts b/fjs/crypto/pow/module.f.ts
@@ -6,7 +6,7 @@
  */
 import { mask } from '../../types/bigint/module.f.ts'
 import { type Vec, uint } from '../../types/bit_vec/module.f.ts'
-import type { Nullable } from '../../types/nullable/module.f.ts'
+import type { Nullable } from '../../types/nullable/module.f.mjs'
 import { computeSync, sha256, type Sha2 } from '../sha2/module.f.ts'
 
 const nBitsMantissa = mask(24n)

diff --git a/fjs/crypto/sha2/proof.f.ts b/fjs/crypto/sha2/proof.f.ts
@@ -1,6 +1,6 @@
 import { utf8 } from '../../text/module.f.ts'
 import { repeat, uint, vec } from '../../types/bit_vec/module.f.ts'
-import { flip } from '../../types/function/module.f.ts'
+import { flip } from '../../types/function/module.f.mjs'
 import { assertEq } from '../../asserts/module.f.mjs'
 import { map } from '../../types/list/module.f.ts'
 import {

diff --git a/fjs/crypto/vdf/module.f.ts b/fjs/crypto/vdf/module.f.ts
@@ -15,7 +15,7 @@
  * ```
  */
 import { modSqrt, prime_field, type PrimeField } from '../../types/prime_field/module.f.ts'
-import type { Nullable } from '../../types/nullable/module.f.ts'
+import type { Nullable } from '../../types/nullable/module.f.mjs'
 import type { Unary } from '../../types/bigint/module.f.ts'
 
 /** Sloth VDF modulus (3072-bit safe prime, same as reference implementations). */

diff --git a/fjs/djs/serializer/module.f.ts b/fjs/djs/serializer/module.f.ts
@@ -10,7 +10,7 @@ import { fold } from '../../types/list/module.f.ts'
 import { concat } from '../../types/string/module.f.ts'
 import { type List, flat, flatMap, map, concat as listConcat } from '../../types/list/module.f.ts'
 const { entries } = Object
-import { compose, fn } from '../../types/function/module.f.ts'
+import { compose, fn } from '../../types/function/module.f.mjs'
 import { serialize as bigintSerialize } from '../../types/bigint/module.f.ts'
 import { objectWrap, arrayWrap, stringSerialize, numberSerialize, nullSerialize, boolSerialize } from '../../media/json/serializer/module.f.ts'
 

diff --git a/fjs/djs/serializer/proof.f.ts b/fjs/djs/serializer/proof.f.ts
@@ -1,6 +1,6 @@
 import { countRefs, stringify, stringifyAsTree } from './module.f.ts'
 import { sort } from '../../types/object/module.f.ts'
-import { identity } from '../../types/function/module.f.ts'
+import { identity } from '../../types/function/module.f.mjs'
 import { setProperty } from '../../media/json/module.f.ts'
 import { assertEq } from '../../asserts/module.f.mjs'
 

diff --git a/fjs/effects/module.f.ts b/fjs/effects/module.f.ts
@@ -98,7 +98,7 @@
 import { assert } from '../asserts/module.f.mjs'
 import { fold, type List } from '../types/list/module.f.ts'
 import { at } from '../types/object/module.f.ts'
-import type { Option } from '../types/option/module.f.ts'
+import type { Option } from '../types/option/module.f.mjs'
 import type { Result } from '../types/result/module.f.ts'
 
 export type Operation =

diff --git a/fjs/fsc/module.f.ts b/fjs/fsc/module.f.ts
@@ -13,7 +13,7 @@ import {
 } from '../types/range_map/module.f.ts'
 import { reduce as listReduce, toArray, map, type List } from '../types/list/module.f.ts'
 import { range as asciiRange } from '../text/ascii/module.f.ts'
-import { flip, fn } from '../types/function/module.f.ts'
+import { flip, fn } from '../types/function/module.f.mjs'
 import { one, type Range } from '../types/range/module.f.ts'
 import { assertEq } from '../asserts/module.f.mjs'
 

diff --git a/fjs/fsm/module.f.ts b/fjs/fsm/module.f.ts
@@ -17,7 +17,7 @@ import {
 } from '../types/range_map/module.f.ts'
 import { type Fold, type Scan, strictEqual } from '../types/function/operator/module.f.ts'
 import { stringify } from '../media/json/module.f.ts'
-import { identity } from '../types/function/module.f.ts'
+import { identity } from '../types/function/module.f.mjs'
 import { stringToList } from '../text/utf16/module.f.ts'
 import { cmp } from '../types/string/module.f.ts'
 

diff --git a/fjs/fsm/proof.f.ts b/fjs/fsm/proof.f.ts
@@ -2,7 +2,7 @@ import { dfa, run, toRange, toUnion, type Grammar } from './module.f.ts'
 import { union } from '../types/byte_set/module.f.ts'
 import { sort, fromEntries } from '../types/object/module.f.ts'
 import { stringify } from '../media/json/module.f.ts'
-import { identity } from '../types/function/module.f.ts'
+import { identity } from '../types/function/module.f.mjs'
 import { toArray } from '../types/list/module.f.ts'
 import { stringToList } from '../text/utf16/module.f.ts'
 import { assertEq } from '../asserts/module.f.mjs'

diff --git a/fjs/mcp/cas/module.f.ts b/fjs/mcp/cas/module.f.ts
@@ -119,7 +119,7 @@ import {
 } from '../../protocol/mcp/module.f.ts'
 import { collectRead, fileCas, type FileCasOperation } from '../../cas/module.f.ts'
 import { fromVec } from '../../text/utf8/module.f.ts'
-import { identity } from '../../types/function/module.f.ts'
+import { identity } from '../../types/function/module.f.mjs'
 import { sha256 } from '../../crypto/sha2/module.f.ts'
 import { nonEmpty, empty as elEmpty } from '../../effects/list/module.f.ts'
 import { syncRevision, type Cache } from '../../cas/evo/module.f.ts'

diff --git a/fjs/mcp/evo/module.f.ts b/fjs/mcp/evo/module.f.ts
@@ -51,7 +51,7 @@ import {
     type ToolEntry, type ToolsCallResult,
 } from '../../protocol/mcp/module.f.ts'
 import { stringify } from '../../media/json/module.f.ts'
-import { identity } from '../../types/function/module.f.ts'
+import { identity } from '../../types/function/module.f.mjs'
 import { type Evo } from '../../cas/evo/module.f.ts'
 
 // ── Argument schemas (declared once, used for both inputSchema and validate) ─────

diff --git a/fjs/media/html/module.f.ts b/fjs/media/html/module.f.ts
@@ -7,7 +7,7 @@
 import { map, flatMap, flat, concat as listConcat, type List } from '../../types/list/module.f.ts'
 import { concat, concat as stringConcat } from '../../types/string/module.f.ts'
 import { definedEntries, type Entry, type StringMap } from '../../types/object/module.f.ts'
-import { compose } from '../../types/function/module.f.ts'
+import { compose } from '../../types/function/module.f.mjs'
 import { stringToList } from '../../text/utf16/module.f.ts'
 import { includes } from '../../types/array/module.f.ts'
 import { type Vec } from '../../types/bit_vec/module.f.ts'

diff --git a/fjs/media/json/module.f.ts b/fjs/media/json/module.f.ts
@@ -18,7 +18,7 @@ import type { Result } from '../../types/result/module.f.ts'
 import { parse as parseTokens } from './parser/module.f.ts'
 import { tokenize } from './tokenizer/module.f.ts'
 import { at, definedEntries, type Entry as ObjectEntry } from '../../types/object/module.f.ts'
-import { compose, fn } from '../../types/function/module.f.ts'
+import { compose, fn } from '../../types/function/module.f.mjs'
 import { objectWrap, arrayWrap, stringSerialize, numberSerialize, nullSerialize, boolSerialize } from './serializer/module.f.ts'
 import { boolean as rttiBoolean, number as rttiNumber, string as rttiString, or, record, array as rttiArray } from '../../types/rtti/module.f.ts'
 import type { Ts } from '../../types/rtti/ts/module.f.ts'

diff --git a/fjs/media/json/proof.f.ts b/fjs/media/json/proof.f.ts
@@ -1,6 +1,6 @@
 import { parse, setProperty, stringify } from './module.f.ts'
 import { sort } from '../../types/object/module.f.ts'
-import { identity } from '../../types/function/module.f.ts'
+import { identity } from '../../types/function/module.f.mjs'
 import { assertEq } from '../../asserts/module.f.mjs'
 
 export const proof = {

diff --git a/fjs/media/type/module.f.ts b/fjs/media/type/module.f.ts
@@ -34,7 +34,7 @@
  */
 import { msb, fromSentinel, length, u8List, type Vec } from '../../types/bit_vec/module.f.ts'
 import { iterable } from '../../types/list/module.f.ts'
-import type { Nullable } from '../../types/nullable/module.f.ts'
+import type { Nullable } from '../../types/nullable/module.f.mjs'
 import { pure, step, type Effect, type Operation } from '../../effects/module.f.ts'
 import type { List } from '../../effects/list/module.f.ts'
 import type { IoResult } from '../../effects/node/module.f.ts'

diff --git a/fjs/sul/level/literal/module.f.ts b/fjs/sul/level/literal/module.f.ts
@@ -7,7 +7,7 @@
 
 import { log2 } from '../../../types/bigint/module.f.ts'
 import { msb, vec, type Vec } from '../../../types/bit_vec/module.f.ts'
-import type { Func } from '../../../types/function/module.f.ts'
+import type { Func } from '../../../types/function/module.f.mjs'
 import { strictEqual, type Equal, type StateScan } from '../../../types/function/operator/module.f.ts'
 import { equal, map, type List } from '../../../types/list/module.f.ts'
 import { join } from '../../../types/string/module.f.ts'

diff --git a/fjs/text/module.f.ts b/fjs/text/module.f.ts
@@ -9,7 +9,7 @@ import { msb, tryU8ListToVec, u8List, type Vec } from '../types/bit_vec/module.f
 import { flatMap, type List } from '../types/list/module.f.ts'
 import { fromCodePointList, toCodePointList } from './utf8/module.f.ts'
 import { stringToCodePointList, codePointListToString } from './utf16/module.f.ts'
-import { mapUnwrap, type Nullable } from '../types/nullable/module.f.ts'
+import { mapUnwrap, type Nullable } from '../types/nullable/module.f.mjs'
 
 export type Block = ItemThunk | ItemArray
 

diff --git a/fjs/text/utf16/module.f.ts b/fjs/text/utf16/module.f.ts
@@ -16,7 +16,7 @@ import {
 } from '../../types/list/module.f.ts'
 import { concat, type StateScan } from '../../types/function/operator/module.f.ts'
 import { contains } from '../../types/range/module.f.ts'
-import { fn } from '../../types/function/module.f.ts'
+import { fn } from '../../types/function/module.f.mjs'
 import {
     decoder,
     errorMask,

diff --git a/fjs/types/array/module.f.ts b/fjs/types/array/module.f.ts
@@ -4,7 +4,7 @@
  * @module
  */
 
-import { fromUndefined, map } from '../nullable/module.f.ts'
+import { fromUndefined, map } from '../nullable/module.f.mjs'
 
 export const isArray = (value: unknown): value is readonly unknown[] =>
     value instanceof Array

diff --git a/fjs/types/bit_vec/module.f.ts b/fjs/types/bit_vec/module.f.ts
@@ -22,13 +22,13 @@
  * @module
  */
 import { bitLength, divUp, mask, maxLength, xor, type Reduce as BigintReduce } from '../bigint/module.f.ts'
-import { flip, identity } from '../function/module.f.ts'
+import { flip, identity } from '../function/module.f.mjs'
 import type { Binary, Fold, Reduce as OpReduce } from '../function/operator/module.f.ts'
 import { map, tryFold, type Accumulator, type List, type Thunk } from '../list/module.f.ts'
 import { asBase, asNominal, type Nominal } from '../nominal/module.f.ts'
 import { repeat as mRepeat } from '../../common/monoid/module.f.ts'
 import { cmp, max, min, type Sign } from '../function/compare/module.f.ts'
-import { mapUnwrap, type Nullable } from '../nullable/module.f.ts'
+import { mapUnwrap, type Nullable } from '../nullable/module.f.mjs'
 
 /**
  * A vector of bits represented as a signed `bigint`.

diff --git a/fjs/types/btree/module.f.ts b/fjs/types/btree/module.f.ts
@@ -4,7 +4,7 @@
  * @module
  */
 import { flat, type List, type Thunk } from '../list/module.f.ts'
-import { map } from '../nullable/module.f.ts'
+import { map } from '../nullable/module.f.mjs'
 import type { TNode, Tree } from './types/module.f.ts'
 
 const nodeValues: <T>(node: TNode<T>) => Thunk<T>

diff --git a/fjs/types/btree/remove/module.f.ts b/fjs/types/btree/remove/module.f.ts
@@ -8,7 +8,7 @@ import type { Compare } from '../../function/compare/module.f.ts'
 import { type Path, type PathItem, find } from '../find/module.f.ts'
 import { fold, concat, next } from '../../list/module.f.ts'
 import type { Array2 } from '../../array/module.f.ts'
-import { map } from '../../nullable/module.f.ts'
+import { map } from '../../nullable/module.f.mjs'
 
 type Leaf01<T> = null | Leaf1<T>
 

diff --git a/fjs/types/byte_set/module.f.ts b/fjs/types/byte_set/module.f.ts
@@ -3,7 +3,7 @@
  *
  * @module
  */
-import { compose } from '../function/module.f.ts'
+import { compose } from '../function/module.f.mjs'
 import type { RangeMap } from '../range_map/module.f.ts'
 import type { SortedSet } from '../sorted_set/module.f.ts'
 import { reverse, countdown, flat, map } from '../list/module.f.ts'

diff --git a/fjs/types/function/module.f.mjs b/fjs/types/function/module.f.mjs
@@ -0,0 +1,47 @@
+/**
+ * A generic function type.
+ *
+ * @template I, O
+ * @typedef {(_: I) => O} Func
+ */
+
+/**
+ * A postfix compose function.
+ *
+ * @type {<I, X>(g: Func<I, X>) => <O>(f: Func<X, O>) => Func<I, O>}
+ */
+export const compose = g => f => x => f(g(x))
+
+/**
+ * A generic identity function.
+ *
+ * @type {<T>(value: T) => T}
+ */
+export const identity = value => value
+
+/**
+ * Flips the arguments of a curried function.
+ *
+ * @type {<A, B, C>(f: (a: A) => (b: B) => C) => (b: B) => (a: A) => C}
+ */
+export const flip = f => b => a => f(a)(b)
+
+/**
+ * A functional utility type that enables seamless chaining of transformations.
+ *
+ * @template I, O
+ * @typedef {{
+ *   readonly result: Func<I, O>
+ *   readonly map: <T>(g: Func<O, T>) => Fn<I, T>
+ * }} Fn
+ */
+
+/**
+ * Creates an `Fn` instance from a function, enabling chaining of transformations.
+ *
+ * @type {<I, O>(result: Func<I, O>) => Fn<I, O>}
+ */
+export const fn = result => ({
+    result,
+    map: g => fn(compose(result)(g))
+})
diff --git a/fjs/types/function/module.f.ts b/fjs/types/function/module.f.ts