Add `step` func as a no-effects substitue for `[]` proc

ehmry/xdg_open_ng#2

preserves.nimble

@@ -1,6 +1,6 @@
 # Package
 
-version       = "3.2.0" # versioned in git, this version is just to confuse nimble
+version       = "3.3.0" # versioned in git, this version is just to confuse nimble
 author        = "Emery Hemingway"
 description   = "data model and serialization format"
 license       = "Unlicense"

src/preserves.nim

@@ -9,7 +9,7 @@ from std/strutils import parseEnum
 import ./preserves/private/dot
 
 when defined(tracePreserves):
-  template trace(args: varargs[untyped]) = stderr.writeLine(args)
+  template trace(args: varargs[untyped]) = {.cast(noSideEffect).}: stderr.writeLine(args)
 else:
   template trace(args: varargs[untyped]) = discard
 
@@ -241,7 +241,8 @@ proc excl*(pr: var Preserve; key: Preserve) =
       break
 
 proc `[]`*(pr, key: Preserve): Preserve =
-  ## Select a value by `key` from the Preserves dictionary `pr`.
+  ## Select a value by `key` from `pr`.
+  ## Works for sequences, records, and dictionaries.
   if pr.isDictionary:
     for (k, v) in pr.dict.items:
       if k == key: return v
@@ -251,6 +252,25 @@ proc `[]`*(pr, key: Preserve): Preserve =
   else:
     raise newException(ValueError, "invalid Preserves indexing")
 
+func step*(pr, idx: Preserve): Option[Preserve] =
+  ## Step into `pr` by index `idx`.
+  ## Works for sequences, records, and dictionaries.
+  runnableExamples:
+    import options
+    import preserves, preserves/parse
+    assert step(parsePreserves("""<foo 1 2>"""), 1.toPreserve) == some(2.toPreserve)
+    assert step(parsePreserves("""{ foo: 1 bar: 2}"""), "foo".toSymbol) == some(1.toPreserve)
+    assert step(parsePreserves("""[ ]"""), 1.toPreserve) == none(Preserve[void])
+  if pr.isDictionary:
+    for (k, v) in pr.dict.items:
+      if k == idx:
+        result = some(v)
+        break
+  elif (pr.isRecord or pr.isSequence) and idx.isInteger:
+    let i = int idx.int
+    if i < pr.len:
+      result = some(pr[i])
+
 proc `[]=`*(pr: var Preserve; key, val: Preserve) =
   ## Insert `val` by `key` in the Preserves dictionary `pr`.
   for i in 0..pr.dict.high: