fix: rename $expand-macro to $eval

Author: tyru

_docs/json-dsl.md

@@ -334,7 +334,7 @@ Macros are not saved in transaction log (expanded before saving).
   * What value is returned depends on each operator's `InvertExpr()`
     implementation.
 
-* `["$expand-macro", expr Value] Value`
+* `["$eval", expr Value] Value`
   * Evaluate `expr` at parsing time.
     This is useful to save evaluated value to transaction log,
     instead of its expression.
@@ -416,9 +416,9 @@ Macros are not saved in transaction log (expanded before saving).
     {target_hash}`
   * It does nothing for bare git repository.
   * e.g.
-    * `["repos/git/update", "github.com/tyru/caw.vim", ["$expand-macro", ["repos/git/rev-parse", "HEAD", "github.com/tyru/caw.vim"]], ["$expand-macro", ["repos/git/fetch", "github.com/tyru/caw.vim"]]]`
+    * `["repos/git/update", "github.com/tyru/caw.vim", ["$eval", ["repos/git/rev-parse", "HEAD", "github.com/tyru/caw.vim"]], ["$eval", ["repos/git/fetch", "github.com/tyru/caw.vim"]]]`
       * To save evaluated hash string in transaction log instead of its
-        expression, apply `$expand-macro` to `repos/git/fetch` expression.
+        expression, apply `$eval` to `repos/git/fetch` expression.
     * `["$invert", ["repos/git/update", path, target_hash, prev_hash]]` = `["repos/git/update", ["$invert", path], ["$invert", prev_hash], ["$invert", target_hash]]`
 
 * `["repos/git/rev-parse", str string, path ReposPath] hash string`

dsl/op/eval.go

@@ -0,0 +1,31 @@
+package op
+
+import (
+	"context"
+
+	"github.com/vim-volt/volt/dsl/types"
+)
+
+func init() {
+	s := evalOp("$eval")
+	EvalOp = &s
+	macroMap[string(*EvalOp)] = EvalOp
+}
+
+type evalOp string
+
+// EvalOp is "$eval" operation
+var EvalOp *evalOp
+
+// String returns "$eval"
+func (*evalOp) String() string {
+	return string(*EvalOp)
+}
+
+// Execute executes "$eval" operation
+func (*evalOp) Expand(args []types.Value) (types.Value, func(), error) {
+	if err := signature(types.AnyValue).check(args); err != nil {
+		return nil, NoRollback, err
+	}
+	return args[0].Eval(context.Background())
+}