diff --git a/src/Ord.ts b/src/Ord.ts
index 6e7b212f8..2f039ae6d 100644
--- a/src/Ord.ts
+++ b/src/Ord.ts
@@ -63,6 +63,7 @@ export const ordBoolean: Ord<boolean> = {
   compare
 }
 
+// TODO: curry in v3
 /**
  * Test whether one value is _strictly less than_ another
  *
@@ -72,6 +73,7 @@ export function lt<A>(O: Ord<A>): (x: A, y: A) => boolean {
   return (x, y) => O.compare(x, y) === -1
 }
 
+// TODO: curry in v3
 /**
  * Test whether one value is _strictly greater than_ another
  *
@@ -81,6 +83,7 @@ export function gt<A>(O: Ord<A>): (x: A, y: A) => boolean {
   return (x, y) => O.compare(x, y) === 1
 }
 
+// TODO: curry in v3
 /**
  * Test whether one value is _non-strictly less than_ another
  *
@@ -90,6 +93,7 @@ export function leq<A>(O: Ord<A>): (x: A, y: A) => boolean {
   return (x, y) => O.compare(x, y) !== 1
 }
 
+// TODO: curry in v3
 /**
  * Test whether one value is _non-strictly greater than_ another
  *
@@ -99,6 +103,7 @@ export function geq<A>(O: Ord<A>): (x: A, y: A) => boolean {
   return (x, y) => O.compare(x, y) !== -1
 }
 
+// TODO: curry in v3
 /**
  * Take the minimum of two values. If they are considered equal, the first argument is chosen
  *
@@ -108,6 +113,7 @@ export function min<A>(O: Ord<A>): (x: A, y: A) => A {
   return (x, y) => (O.compare(x, y) === 1 ? y : x)
 }
 
+// TODO: curry in v3
 /**
  * Take the maximum of two values. If they are considered equal, the first argument is chosen
  *
diff --git a/src/ReadonlyArray.ts b/src/ReadonlyArray.ts
index f369e5f67..803b18676 100644
--- a/src/ReadonlyArray.ts
+++ b/src/ReadonlyArray.ts
@@ -1094,7 +1094,7 @@ export function elem<A>(
   (a: A, as: ReadonlyArray<A>): boolean
 }
 export function elem<A>(E: Eq<A>): (a: A, as?: ReadonlyArray<A>) => boolean | ((as: ReadonlyArray<A>) => boolean) {
-  return (a, as) => {
+  return (a, as?) => {
     if (as === undefined) {
       const elemE = elem(E)
       return (as) => elemE(a, as)
@@ -1329,7 +1329,7 @@ export function union<A>(
   E: Eq<A>
 ): (xs: ReadonlyArray<A>, ys?: ReadonlyArray<A>) => ReadonlyArray<A> | ((ys: ReadonlyArray<A>) => ReadonlyArray<A>) {
   const elemE = elem(E)
-  return (xs, ys) => {
+  return (xs, ys?) => {
     if (ys === undefined) {
       const unionE = union(E)
       return (ys) => unionE(ys, xs)
@@ -1366,7 +1366,7 @@ export function intersection<A>(
   E: Eq<A>
 ): (xs: ReadonlyArray<A>, ys?: ReadonlyArray<A>) => ReadonlyArray<A> | ((ys: ReadonlyArray<A>) => ReadonlyArray<A>) {
   const elemE = elem(E)
-  return (xs, ys) => {
+  return (xs, ys?) => {
     if (ys === undefined) {
       const intersectionE = intersection(E)
       return (ys) => intersectionE(ys, xs)
@@ -1400,7 +1400,7 @@ export function difference<A>(
   E: Eq<A>
 ): (xs: ReadonlyArray<A>, ys?: ReadonlyArray<A>) => ReadonlyArray<A> | ((ys: ReadonlyArray<A>) => ReadonlyArray<A>) {
   const elemE = elem(E)
-  return (xs, ys) => {
+  return (xs, ys?) => {
     if (ys === undefined) {
       const differenceE = difference(E)
       return (ys) => differenceE(ys, xs)
diff --git a/src/ReadonlyMap.ts b/src/ReadonlyMap.ts
index fc94c5d8b..d54e6432e 100644
--- a/src/ReadonlyMap.ts
+++ b/src/ReadonlyMap.ts
@@ -93,7 +93,7 @@ export function member<K>(
 }
 export function member<K>(E: Eq<K>): <A>(k: K, m?: ReadonlyMap<K, A>) => boolean | ((m: ReadonlyMap<K, A>) => boolean) {
   const lookupE = lookup(E)
-  return (k, m) => {
+  return (k, m?) => {
     if (m === undefined) {
       const memberE = member(E)
       return (m) => memberE(k, m)
@@ -120,7 +120,7 @@ export function elem<A>(
   <K>(a: A, m: ReadonlyMap<K, A>): boolean
 }
 export function elem<A>(E: Eq<A>): <K>(a: A, m?: ReadonlyMap<K, A>) => boolean | ((m: ReadonlyMap<K, A>) => boolean) {
-  return (a, m) => {
+  return (a, m?) => {
     if (m === undefined) {
       const elemE = elem(E)
       return (m) => elemE(a, m)
@@ -349,7 +349,7 @@ export function lookup<K>(
   E: Eq<K>
 ): <A>(k: K, m?: ReadonlyMap<K, A>) => Option<A> | ((m: ReadonlyMap<K, A>) => Option<A>) {
   const lookupWithKeyE = lookupWithKey(E)
-  return (k, m) => {
+  return (k, m?) => {
     if (m === undefined) {
       const lookupE = lookup(E)
       return (m) => lookupE(k, m)
diff --git a/src/Semigroup.ts b/src/Semigroup.ts
index 93f9e54b2..998db1b7c 100644
--- a/src/Semigroup.ts
+++ b/src/Semigroup.ts
@@ -34,7 +34,7 @@ export function fold<A>(
   (a: A, as: ReadonlyArray<A>): A
 }
 export function fold<A>(S: Semigroup<A>): (a: A, as?: ReadonlyArray<A>) => A | ((as: ReadonlyArray<A>) => A) {
-  return (a, as) => {
+  return (a, as?) => {
     if (as === undefined) {
       const foldS = fold(S)
       return (as) => foldS(a, as)
diff --git a/src/Tree.ts b/src/Tree.ts
index 109f3c182..3ed361b51 100644
--- a/src/Tree.ts
+++ b/src/Tree.ts
@@ -208,6 +208,7 @@ export function unfoldForestM<M>(
     )
 }
 
+// TODO: curry in v3
 /**
  * @since 2.0.0
  */