Add dhall-cache-url to support using a dhall cache

.github/actions/spelling/expect.txt

@@ -3,4 +3,6 @@ endgroup
 jsoref
 Linting
 mktemp
+tarball
 workflows
+XDG

action.yml

@@ -26,6 +26,9 @@ inputs:
   dhall-file-list:
     description: 'Null delimited file of dhall files to check'
     required: false
+  dhall-cache-url:
+    description: 'Dhall cache tarball'
+    required: false
   parallel-jobs:
     description: 'Number of CPUs available for running checks'
     required: false
@@ -47,6 +50,7 @@ runs:
   steps:
     - shell: bash
       env:
+        DHALL_CACHE_URL: ${{ inputs.dhall-cache-url }}
         DHALL_HASKELL: ${{ inputs.dhall-haskell }}
         DHALL_HASKELL_BINARY: ${{ inputs.dhall-haskell-binary }}
         DHALL_JSON: ${{ inputs.dhall-json }}

check.sh

@@ -1,4 +1,7 @@
 #!/bin/sh -e
+if [ -n "$VERBOSE" ]; then
+  set -x
+fi
 
 dhall_json="$(eval echo "$DHALL_JSON_BINARY")"
 dhall_haskell="$(eval echo "$DHALL_HASKELL_BINARY")"
@@ -17,6 +20,30 @@ PATH="$dhall_haskell_extracted/bin:$PATH"
 
 echo "::add-matcher::$GITHUB_ACTION_PATH/dhall-checker.json"
 
+if [ -z "$XDG_CACHE_HOME" ]; then
+  export XDG_CACHE_HOME=/tmp/dhall-cache
+  mkdir -p "$XDG_CACHE_HOME"
+  echo "XDG_CACHE_HOME=$XDG_CACHE_HOME" >> "$GITHUB_ENV"
+fi
+
+if [ -n "$DHALL_CACHE_URL" ]; then
+  dhall_cache=$(mktemp)
+  curl -sSL "$DHALL_CACHE_URL" -o "$dhall_cache"
+  (
+    cd "$XDG_CACHE_HOME"
+    tar xf "$dhall_cache"
+    root_items="$(
+      tar tf "$dhall_cache" |
+      perl -pe 's!/.*!!' |
+      uniq
+    )"
+    if [ $(echo "$root_items" | wc -l) = 1 ] ; then
+      mv "$root_items"/* .
+      rmdir "$root_items"
+    fi
+  )
+fi
+
 export DHALL_FAILURES=$(mktemp -d)
 if [ -z "$LIST" ]; then
   export LIST=$(mktemp)