Improve UX of fetch-params

.circleci/config.yml

@@ -45,7 +45,7 @@ commands:
             - 'v25-2k-lotus-params'
           paths:
             - /var/tmp/filecoin-proof-parameters/
-      - run:  ./lotus fetch-params --proving-params 2048
+      - run:  ./lotus fetch-params 2048
       - save_cache:
           name: Save parameters cache
           key: 'v25-2k-lotus-params'

cli/params.go

@@ -10,18 +10,16 @@ import (
 )
 
 var fetchParamCmd = &cli.Command{
-	Name:  "fetch-params",
-	Usage: "Fetch proving parameters",
-	Flags: []cli.Flag{
-		&cli.StringFlag{
-			Name:  "proving-params",
-			Usage: "download params used creating proofs for given size, i.e. 32GiB",
-		},
-	},
+	Name:      "fetch-params",
+	Usage:     "Fetch proving parameters",
+	ArgsUsage: "[sectorSize]",
 	Action: func(cctx *cli.Context) error {
-		sectorSizeInt, err := units.RAMInBytes(cctx.String("proving-params"))
+		if !cctx.Args().Present() {
+			return xerrors.Errorf("must pass sector size to fetch params for (specify as \"32GiB\", for instance)")
+		}
+		sectorSizeInt, err := units.RAMInBytes(cctx.Args().First())
 		if err != nil {
-			return err
+			return xerrors.Errorf("error parsing sector size (specify as \"32GiB\", for instance): %w", err)
 		}
 		sectorSize := uint64(sectorSizeInt)