diff --git a/src/services/generateKeys.ts b/src/services/generateKeys.ts
index 842c2a7..5e01f74 100644
--- a/src/services/generateKeys.ts
+++ b/src/services/generateKeys.ts
@@ -1,6 +1,4 @@
-interface GenerateKeys {
-  (startKey: number, endKey: number): number[]
-}
+type GenerateKeys = (startKey: number, endKey: number) => number[]
 
 const generateKeys: GenerateKeys = (startKey, endKey) =>
   Array(endKey - startKey + 1)
diff --git a/src/services/getFractionalOctaveCount.ts b/src/services/getFractionalOctaveCount.ts
index 787d0fb..5576ee2 100644
--- a/src/services/getFractionalOctaveCount.ts
+++ b/src/services/getFractionalOctaveCount.ts
@@ -6,9 +6,7 @@ import * as caches from './caches'
 
 const getOctaveCompleteness = mem(getOctaveCompletenessUnmemoized, { cache: caches.getOctaveCompleteness })
 
-interface GetFractionalOctaveCount {
-  (startKey: number, endKey: number): number
-}
+type GetFractionalOctaveCount = (startKey: number, endKey: number) => number
 
 const getFractionalOctaveCount: GetFractionalOctaveCount = (startKey, endKey) => {
   const dummyKeys = generateKeys(startKey, endKey)
diff --git a/src/services/getKeyLeft.ts b/src/services/getKeyLeft.ts
index e3025ae..2082dc2 100644
--- a/src/services/getKeyLeft.ts
+++ b/src/services/getKeyLeft.ts
@@ -10,15 +10,9 @@ const getOctaveCount = mem(getOctaveCountUnmemoized, { cacheKey: (args) => args.
 const getFractionalOctaveCount = mem(getFractionalOctaveCountUnmemoized, { cacheKey: (args) => args.join(':') })
 const getKeyOctave = mem(getKeyOctaveUnmemoized, { cache: caches.getKeyOctave })
 
-export interface GetKeyLeft {
-  (k: number): number
-}
-
-interface GetKeyLeftDecorator {
-  (startKey: number, endKey: number): GetKeyLeft
-}
+type GetKeyLeft = (startKey: number, endKey: number) => (k: number) => number
 
-const getKeyLeftDecorator: GetKeyLeftDecorator = (startKey, endKey): GetKeyLeft => (k) => {
+const getKeyLeft: GetKeyLeft = (startKey, endKey) => (k) => {
   const fractionalOctaveCount = getFractionalOctaveCount(startKey, endKey)
   const octaveCount = getOctaveCount(startKey, endKey)
   const startOctave = getKeyOctave(startKey)
@@ -29,4 +23,4 @@ const getKeyLeftDecorator: GetKeyLeftDecorator = (startKey, endKey): GetKeyLeft
   return theKeyOffset - firstKeyOffset
 }
 
-export default getKeyLeftDecorator
+export default getKeyLeft
diff --git a/src/services/getKeyOctave.ts b/src/services/getKeyOctave.ts
index 8222caf..38959c0 100644
--- a/src/services/getKeyOctave.ts
+++ b/src/services/getKeyOctave.ts
@@ -1,6 +1,4 @@
-interface GetKeyOctave {
-  (k: number): number
-}
+type GetKeyOctave = (k: number) => number
 
 const getKeyOctave: GetKeyOctave = (k) => Math.floor(k / 12)
 
diff --git a/src/services/getKeyWidth.ts b/src/services/getKeyWidth.ts
index 2d71b71..fc75001 100644
--- a/src/services/getKeyWidth.ts
+++ b/src/services/getKeyWidth.ts
@@ -5,13 +5,7 @@ import getFractionalOctaveCountUnmemoized from './getFractionalOctaveCount'
 import { ACCIDENTAL_KEY_TO_NATURAL_KEY_WIDTH_RATIO } from './constants'
 import * as caches from './caches'
 
-export interface GetKeyWidth {
-  (k: number): number
-}
-
-interface GetKeyWidthDecorator {
-  (startKey: number, endKey: number): GetKeyWidth
-}
+type GetKeyWidthDecorator = (startKey: number, endKey: number) => (k: number) => number
 
 const isNaturalKey = mem(isNaturalKeyUnmemoized, { cache: caches.isNaturalKey })
 const getFractionalOctaveCount = mem(getFractionalOctaveCountUnmemoized, {
@@ -19,11 +13,11 @@ const getFractionalOctaveCount = mem(getFractionalOctaveCountUnmemoized, {
 })
 const getOctaveCount = mem(getOctaveCountUnmemoized, { cacheKey: (args) => args.join(':') })
 
-const getKeyWidthDecorator: GetKeyWidthDecorator = (startKey, endKey) => (k) => {
+const getKeyWidth: GetKeyWidthDecorator = (startKey, endKey) => (k) => {
   const fractionalOctaveCount = getFractionalOctaveCount(startKey, endKey)
   const octaveCount = getOctaveCount(startKey, endKey)
   const naturalKeyWidth = (100 * (octaveCount / fractionalOctaveCount)) / (octaveCount * 7)
   return isNaturalKey(k) ? naturalKeyWidth : naturalKeyWidth * ACCIDENTAL_KEY_TO_NATURAL_KEY_WIDTH_RATIO
 }
 
-export default getKeyWidthDecorator
+export default getKeyWidth
diff --git a/src/services/getKeyXOffset.ts b/src/services/getKeyXOffset.ts
index 2dea681..bbf2e9c 100644
--- a/src/services/getKeyXOffset.ts
+++ b/src/services/getKeyXOffset.ts
@@ -1,8 +1,6 @@
 import { KEY_OFFSETS } from './constants'
 
-interface GetKeyXOffset {
-  (k: number): number
-}
+type GetKeyXOffset = (k: number) => number
 
 const getKeyXOffset: GetKeyXOffset = (k) => {
   return KEY_OFFSETS[k % 12]
diff --git a/src/services/getOctaveCompleteness.ts b/src/services/getOctaveCompleteness.ts
index f25e5c9..67c8ab1 100644
--- a/src/services/getOctaveCompleteness.ts
+++ b/src/services/getOctaveCompleteness.ts
@@ -6,9 +6,7 @@ import isNaturalKeyUnmemoized from './isNaturalKey'
 const getKeyXOffset = mem(getKeyXOffsetUnmemoized, { cache: caches.getKeyXOffset })
 const isNaturalKey = mem(isNaturalKeyUnmemoized, { cache: caches.isNaturalKey })
 
-interface GetOctaveCompleteness {
-  (firstKey: number, lastKey: number): number
-}
+type GetOctaveCompleteness = (firstKey: number, lastKey: number) => number
 
 // expect firstKey and lastKey within the same octave
 const getOctaveCompleteness: GetOctaveCompleteness = (firstKey, lastKey) =>
diff --git a/src/services/getOctaveCount.ts b/src/services/getOctaveCount.ts
index 6902786..32df5b0 100644
--- a/src/services/getOctaveCount.ts
+++ b/src/services/getOctaveCount.ts
@@ -4,9 +4,7 @@ import * as caches from './caches'
 
 const getKeyOctave = mem(getKeyOctaveUnmemoized, { cache: caches.getKeyOctave })
 
-interface GetOctaveCount {
-  (startKey: number, endKey: number): number
-}
+type GetOctaveCount = (startKey: number, endKey: number) => number
 
 const getOctaveCount: GetOctaveCount = (startKey, endKey) => getKeyOctave(endKey) - getKeyOctave(startKey) + 1
 
diff --git a/src/services/groupKeysIntoOctaves.ts b/src/services/groupKeysIntoOctaves.ts
index 4c6bc0e..958ed8f 100644
--- a/src/services/groupKeysIntoOctaves.ts
+++ b/src/services/groupKeysIntoOctaves.ts
@@ -1,6 +1,4 @@
-interface GroupKeysIntoOctaves {
-  (dummyKeys: number[]): Record<number, number[]>
-}
+type GroupKeysIntoOctaves = (dummyKeys: number[]) => Record<number, number[]>
 
 const groupKeysIntoOctaves: GroupKeysIntoOctaves = (dummyKeys) =>
   dummyKeys
diff --git a/src/services/isNaturalKey.ts b/src/services/isNaturalKey.ts
index ad50c45..e88e91d 100644
--- a/src/services/isNaturalKey.ts
+++ b/src/services/isNaturalKey.ts
@@ -1,8 +1,6 @@
 const NATURAL_KEYS = [0, 2, 4, 5, 7, 9, 11]
 
-interface IsNaturalKey {
-  (k: number): boolean
-}
+type IsNaturalKey = (k: number) => boolean
 
 const isNaturalKey: IsNaturalKey = (k: number): boolean => {
   const type = typeof (k as unknown)