diff --git a/src/README.md b/src/README.md
index 1010880..f03bbf7 100644
--- a/src/README.md
+++ b/src/README.md
@@ -116,6 +116,8 @@ See also the [tooltip example](../../examples/tooltip/).
 
 @hoverTooltip
 
+@HoverTooltipSource
+
 @hasHoverTooltips
 
 @closeHoverTooltips
diff --git a/src/index.ts b/src/index.ts
index 7b5898f..ab22629 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -15,7 +15,7 @@ export {Rect} from "./dom"
 export {layer, LayerMarker, RectangleMarker} from "./layer"
 export {MatchDecorator} from "./matchdecorator"
 export {rectangularSelection, crosshairCursor} from "./rectangular-selection"
-export {showTooltip, Tooltip, TooltipView, tooltips, getTooltip, hoverTooltip, hasHoverTooltips,
+export {showTooltip, Tooltip, TooltipView, tooltips, getTooltip, hoverTooltip, HoverTooltipSource, hasHoverTooltips,
         closeHoverTooltips, repositionTooltips} from "./tooltip"
 export {showPanel, PanelConstructor, Panel, getPanel, panels} from "./panel"
 export {lineNumbers, highlightActiveLineGutter, gutter, gutters, GutterMarker,
diff --git a/src/tooltip.ts b/src/tooltip.ts
index 948fd16..d10cb36 100644
--- a/src/tooltip.ts
+++ b/src/tooltip.ts
@@ -591,7 +591,9 @@ const showHoverTooltipHost = showTooltip.compute([showHoverTooltip], state => {
 
 const enum Hover { Time = 300, MaxDist = 6 }
 
-type HoverSource = (view: EditorView, pos: number, side: -1 | 1) => Tooltip | readonly Tooltip[] | null | Promise<Tooltip | readonly Tooltip[] | null>
+/// The type of function that can be used as a [hover tooltip
+/// source](#view.hoverTooltip^source).
+export type HoverTooltipSource = (view: EditorView, pos: number, side: -1 | 1) => Tooltip | readonly Tooltip[] | null | Promise<Tooltip | readonly Tooltip[] | null>
 
 class HoverPlugin {
   lastMove: {x: number, y: number, target: HTMLElement, time: number}
@@ -600,7 +602,7 @@ class HoverPlugin {
   pending: {pos: number} | null = null
 
   constructor(readonly view: EditorView,
-              readonly source: HoverSource,
+              readonly source: HoverTooltipSource,
               readonly field: StateField<readonly Tooltip[]>,
               readonly setHover: StateEffectType<readonly Tooltip[]>,
               readonly hoverTime: number) {
@@ -746,7 +748,7 @@ function isOverRange(view: EditorView, from: number, to: number, x: number, y: n
 /// container element. This allows multiple tooltips over the same
 /// range to be "merged" together without overlapping.
 export function hoverTooltip(
-  source: HoverSource,
+  source: HoverTooltipSource,
   options: {
     /// Controls whether a transaction hides the tooltip. The default
     /// is to not hide.