If the user provides default as the value of the arch input, use the runner machine's architecture

action.yml

@@ -1,7 +1,7 @@
 name: 'Setup Julia environment'
 description: 'Setup a Julia environment and add it to the PATH'
 author: 'Sascha Mann'
-inputs: 
+inputs:
   version:
     description: 'The Julia version to download (if necessary) and use. Example: 1.0.4'
     default: '1'
@@ -12,7 +12,7 @@ inputs:
   arch:
     description: 'Architecture of the Julia binaries. Defaults to the architecture of the runner executing the job.'
     required: false
-    default: '${{ runner.arch }}'
+    default: 'default'
   show-versioninfo:
     description: 'Display InteractiveUtils.versioninfo() after installing'
     required: false

src/setup-julia.ts

@@ -3,17 +3,19 @@ import * as tc from '@actions/tool-cache'
 
 import * as fs from 'fs'
 import * as https from 'https'
+import * as os from 'os'
 import * as path from 'path'
 
 import * as installer from './installer'
 
+// Note: before we index into this dict, we always first do `.toLowerCase()` on
+// the key.
+//
+// Therefore, this dict does not need to account for differences in case.
 const archSynonyms = {
     'x86': 'x86',
-    'X86': 'x86',
     'x64': 'x64',
-    'X64': 'x64',
     'aarch64': 'aarch64',
-    'ARM64': 'aarch64',
     'arm64': 'aarch64'
 }
 
@@ -37,24 +39,37 @@ async function run() {
             })
         }
 
-        // Inputs
-        const versionInput = core.getInput('version')
-        const includePrereleases = core.getInput('include-all-prereleases') == 'true'
-        const originalArchInput = core.getInput('arch')
+        // Inputs.
+        // Note that we intentionally strip leading and lagging whitespace by using `.trim()`
+        const versionInput = core.getInput('version').trim()
+        const includePrereleases = core.getInput('include-all-prereleases').trim() == 'true'
+        const originalArchInput = core.getInput('arch').trim()
 
         // It can easily happen that, for example, a workflow file contains an input `version: ${{ matrix.julia-version }}`
         // while the strategy matrix only contains a key `${{ matrix.version }}`.
         // In that case, we want the action to fail, rather than trying to download julia from an URL that's missing parts and 404ing.
         // We _could_ fall back to the default but that means that builds silently do things differently than they're meant to, which
         // is worse than failing the build.
-        if (!versionInput) {
+        if (!versionInput) { // if `versionInput` is an empty string
             throw new Error('Version input must not be null')
         }
-        if (!originalArchInput) {
+        if (!originalArchInput) { // if `originalArchInput` is an empty string
             throw new Error(`Arch input must not be null`)
         }
 
-        const arch = archSynonyms[originalArchInput]
+        let processedArchInput: string;
+        if (originalArchInput == "default") {
+            // If the user sets the `arch` input to `default`, then we use the
+            // architecture of the machine that we are running on.
+            processedArchInput = os.arch();
+            core.debug(`The "arch" input is "default", so we will use the machine arch: ${processedArchInput}`)
+        } else {
+            processedArchInput = originalArchInput;
+        }
+        // Note: we convert the key `processedArchInput` to lower case
+        // before we index into the `archSynonyms` dict.
+        const arch = archSynonyms[processedArchInput.toLowerCase()]
+        core.debug(`Mapped the "arch" from ${processedArchInput} to ${arch}`)
 
         const versionInfo = await installer.getJuliaVersionInfo()
         const availableReleases = await installer.getJuliaVersions(versionInfo)