Address review comments; Add a test

beacon_chain/spec/datatypes.nim

@@ -678,8 +678,18 @@ import json_serialization
 export json_serialization
 export writeValue, readValue
 
+const
+  # http://facweb.cs.depaul.edu/sjost/it212/documents/ascii-pr.htm
+  PrintableAsciiChars = {'!'..'~'}
+
 func `$`*(value: GraffitiBytes): string =
-  strip(string.fromBytes(distinctBase value), chars = Whitespace + {'\0'})
+  result = strip(string.fromBytes(distinctBase value),
+                 leading = false,
+                 chars = Whitespace + {'\0'})
+
+  # TODO: Perhaps handle UTF-8 at some point
+  if not allCharsInSet(result, PrintableAsciiChars):
+    result = "0x" & toHex(distinctBase value)
 
 func init*(T: type GraffitiBytes, input: string): GraffitiBytes
           {.raises: [ValueError, Defect].} =

tests/all_tests.nim

@@ -17,6 +17,7 @@ import # Unit test
   ./test_beaconstate,
   ./test_bitseqs,
   ./test_block_pool,
+  ./test_datatypes,
   ./test_helpers,
   ./test_keystore,
   ./test_mocking,

tests/test_datatypes.nim

@@ -0,0 +1,31 @@
+# beacon_chain
+# Copyright (c) 2018 Status Research & Development GmbH
+# Licensed and distributed under either of
+#   * MIT license (license terms in the root directory or at https://opensource.org/licenses/MIT).
+#   * Apache v2 license (license terms in the root directory or at https://www.apache.org/licenses/LICENSE-2.0).
+# at your option. This file may not be copied, modified, or distributed except according to those terms.
+
+{.used.}
+
+import
+  unittest, typetraits,
+  ../beacon_chain/spec/datatypes,
+  ./testutil
+
+suiteReport "Spec datatypes":
+  timedTest "Graffiti bytes":
+    var
+      g1 = GraffitiBytes.init "Hello"
+      g2 = default(GraffitiBytes)
+      g3 = g2
+
+    distinctBase(g3)[2] = byte(6)
+
+    check:
+      $g1 == "Hello"
+      $g2 == ""
+      $g3 == "0x0000060000000000000000000000000000000000000000000000000000000000"
+
+      g2 == GraffitiBytes.init("")
+      g3 == GraffitiBytes.init($g3)
+