Move opacity modifier support into plugin theme() function (#14348)

This PR moves support for opacity modifies from the CSS `theme()`
function into the plugin `theme()` implementation, this will allow
plugins to use this, too:

```ts
let plugin = plugin(function ({ addUtilities, theme }) {
    addUtilities({
      '.percentage': {
        color: theme('colors.red.500 / 50%'),
      },
      '.fraction': {
        color: theme('colors.red.500 / 0.5'),
      },
      '.variable': {
        color: theme('colors.red.500 / var(--opacity)'),
      },
    })
  })
}
```

There's a small behavioral change for the CSS `theme()` function. Since
tuples are resolved by default for the CSS `theme()` function only,
these will no longer have opacity applied to their first values. This is
probably fine given the reduced complexity as I don't expect the first
values of tuples to be colors and the fix would mean we would have to
parse the modifier in different places.

CHANGELOG.md

@@ -7,7 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
-- Nothing yet!
+### Added
+
+- Add opacity modifier support to the `theme()` function in plugins ([#14348](https://github.com/tailwindlabs/tailwindcss/pull/14348))
 
 ## [4.0.0-alpha.22] - 2024-09-04
 

packages/tailwindcss/src/compat/config/resolve-config.ts

@@ -1,6 +1,6 @@
 import type { DesignSystem } from '../../design-system'
 import type { PluginWithConfig } from '../../plugin-api'
-import { createThemeFn } from '../../theme-fn'
+import { createThemeFn } from '../plugin-functions'
 import { deepMerge, isPlainObject } from './deep-merge'
 import {
   type ResolvedConfig,

packages/tailwindcss/src/compat/plugin-functions.ts

@@ -1,9 +1,10 @@
-import { deepMerge } from './compat/config/deep-merge'
-import type { UserConfig } from './compat/config/types'
-import type { DesignSystem } from './design-system'
-import type { Theme, ThemeKey } from './theme'
-import { DefaultMap } from './utils/default-map'
-import { toKeyPath } from './utils/to-key-path'
+import type { DesignSystem } from '../design-system'
+import type { Theme, ThemeKey } from '../theme'
+import { withAlpha } from '../utilities'
+import { DefaultMap } from '../utils/default-map'
+import { toKeyPath } from '../utils/to-key-path'
+import { deepMerge } from './config/deep-merge'
+import type { UserConfig } from './config/types'
 
 export function createThemeFn(
   designSystem: DesignSystem,
@@ -11,21 +12,40 @@ export function createThemeFn(
   resolveValue: (value: any) => any,
 ) {
   return function theme(path: string, defaultValue?: any) {
-    let keypath = toKeyPath(path)
-    let cssValue = readFromCss(designSystem.theme, keypath)
-
-    if (typeof cssValue !== 'object') {
-      return cssValue
+    // Extract an eventual modifier from the path. e.g.:
+    // - "colors.red.500 / 50%" -> "50%"
+    // - "foo/bar/baz/50%"      -> "50%"
+    let lastSlash = path.lastIndexOf('/')
+    let modifier: string | null = null
+    if (lastSlash !== -1) {
+      modifier = path.slice(lastSlash + 1).trim()
+      path = path.slice(0, lastSlash).trim()
     }
 
-    let configValue = resolveValue(get(configTheme() ?? {}, keypath) ?? null)
+    let resolvedValue = (() => {
+      let keypath = toKeyPath(path)
+      let cssValue = readFromCss(designSystem.theme, keypath)
 
-    if (configValue !== null && typeof configValue === 'object' && !Array.isArray(configValue)) {
-      return deepMerge({}, [configValue, cssValue], (_, b) => b)
+      if (typeof cssValue !== 'object') {
+        return cssValue
+      }
+
+      let configValue = resolveValue(get(configTheme() ?? {}, keypath) ?? null)
+
+      if (configValue !== null && typeof configValue === 'object' && !Array.isArray(configValue)) {
+        return deepMerge({}, [configValue, cssValue], (_, b) => b)
+      }
+
+      // Values from CSS take precedence over values from the config
+      return cssValue ?? configValue
+    })()
+
+    // Apply the opacity modifier if present
+    if (modifier && typeof resolvedValue === 'string') {
+      resolvedValue = withAlpha(resolvedValue, modifier)
     }
 
-    // Values from CSS take precedence over values from the config
-    return cssValue ?? configValue ?? defaultValue
+    return resolvedValue ?? defaultValue
   }
 }
 

packages/tailwindcss/src/css-functions.ts

@@ -1,6 +1,5 @@
 import { walk, type AstNode } from './ast'
 import type { PluginAPI } from './plugin-api'
-import { withAlpha } from './utilities'
 import * as ValueParser from './value-parser'
 import { type ValueAstNode } from './value-parser'
 
@@ -77,27 +76,19 @@ function cssThemeFn(
   path: string,
   fallbackValues: ValueAstNode[],
 ): ValueAstNode[] {
-  let modifier: string | null = null
-  // Extract an eventual modifier from the path. e.g.:
-  // - "colors.red.500 / 50%" -> "50%"
-  // - "foo/bar/baz/50%"      -> "50%"
-  let lastSlash = path.lastIndexOf('/')
-  if (lastSlash !== -1) {
-    modifier = path.slice(lastSlash + 1).trim()
-    path = path.slice(0, lastSlash).trim()
-  }
-
   let resolvedValue: string | null = null
   let themeValue = pluginApi.theme(path)
 
-  if (Array.isArray(themeValue) && themeValue.length === 2) {
+  let isArray = Array.isArray(themeValue)
+  if (isArray && themeValue.length === 2) {
     // When a tuple is returned, return the first element
     resolvedValue = themeValue[0]
-    // We otherwise only ignore string values here, objects (and namespace maps)
-    // are treated as non-resolved values for the CSS `theme()` function.
-  } else if (Array.isArray(themeValue)) {
+  } else if (isArray) {
+    // Arrays get serialized into a comma-separated lists
     resolvedValue = themeValue.join(', ')
   } else if (typeof themeValue === 'string') {
+    // Otherwise only allow string values here, objects (and namespace maps)
+    // are treated as non-resolved values for the CSS `theme()` function.
     resolvedValue = themeValue
   }
 
@@ -107,14 +98,10 @@ function cssThemeFn(
 
   if (!resolvedValue) {
     throw new Error(
-      `Could not resolve value for theme function: \`theme(${path}${modifier ? ` / ${modifier}` : ''})\`. Consider checking if the path is correct or provide a fallback value to silence this error.`,
+      `Could not resolve value for theme function: \`theme(${path})\`. Consider checking if the path is correct or provide a fallback value to silence this error.`,
     )
   }
 
-  if (modifier) {
-    resolvedValue = withAlpha(resolvedValue, modifier)
-  }
-
   // We need to parse the values recursively since this can resolve with another
   // `theme()` function definition.
   return ValueParser.parse(resolvedValue)

packages/tailwindcss/src/index.ts

@@ -3,9 +3,9 @@ import { substituteAtApply } from './apply'
 import { comment, decl, rule, toCss, walk, WalkAction, type Rule } from './ast'
 import type { UserConfig } from './compat/config/types'
 import { compileCandidates } from './compile'
+import { substituteFunctions, THEME_FUNCTION_INVOCATION } from './css-functions'
 import * as CSS from './css-parser'
 import { buildDesignSystem, type DesignSystem } from './design-system'
-import { substituteFunctions, THEME_FUNCTION_INVOCATION } from './functions'
 import { registerPlugins, type CssPluginOptions, type Plugin } from './plugin-api'
 import { Theme } from './theme'
 import { segment } from './utils/segment'

packages/tailwindcss/src/plugin-api.test.ts

@@ -208,6 +208,49 @@ describe('theme', async () => {
     `)
   })
 
+  test('plugin theme can have opacity modifiers', async ({ expect }) => {
+    let input = css`
+      @tailwind utilities;
+      @theme {
+        --color-red-500: #ef4444;
+      }
+      @plugin "my-plugin";
+    `
+
+    let compiler = await compile(input, {
+      loadPlugin: async () => {
+        return plugin(function ({ addUtilities, theme }) {
+          addUtilities({
+            '.percentage': {
+              color: theme('colors.red.500 / 50%'),
+            },
+            '.fraction': {
+              color: theme('colors.red.500 / 0.5'),
+            },
+            '.variable': {
+              color: theme('colors.red.500 / var(--opacity)'),
+            },
+          })
+        })
+      },
+    })
+
+    expect(compiler.build(['percentage', 'fraction', 'variable'])).toMatchInlineSnapshot(`
+      ".fraction {
+        color: color-mix(in srgb, #ef4444 50%, transparent);
+      }
+      .percentage {
+        color: color-mix(in srgb, #ef4444 50%, transparent);
+      }
+      .variable {
+        color: color-mix(in srgb, #ef4444 calc(var(--opacity) * 100%), transparent);
+      }
+      :root {
+        --color-red-500: #ef4444;
+      }
+      "
+    `)
+  })
   test('theme value functions are resolved correctly regardless of order', async ({ expect }) => {
     let input = css`
       @tailwind utilities;
@@ -354,7 +397,7 @@ describe('theme', async () => {
       `)
   })
 
-  test('CSS theme values are mreged with JS theme values', async ({ expect }) => {
+  test('CSS theme values are merged with JS theme values', async ({ expect }) => {
     let input = css`
       @tailwind utilities;
       @plugin "my-plugin";

packages/tailwindcss/src/plugin-api.ts

@@ -6,8 +6,8 @@ import { createCompatConfig } from './compat/config/create-compat-config'
 import { resolveConfig, type ConfigFile } from './compat/config/resolve-config'
 import type { ResolvedConfig, UserConfig } from './compat/config/types'
 import { darkModePlugin } from './compat/dark-mode'
+import { createThemeFn } from './compat/plugin-functions'
 import type { DesignSystem } from './design-system'
-import { createThemeFn } from './theme-fn'
 import { withAlpha, withNegative } from './utilities'
 import { inferDataType } from './utils/infer-data-type'
 import { segment } from './utils/segment'