results: Add Opt/Result converters (#177)

Add `optError`, `optValue`, to convert back and forth between Opt and Result
These conversions end up being more common than others since "trivial"  success/fail-style API often use Opt while combining such API into combined operations tends to prefer richer error reporting.

stew/results.nim

@@ -845,6 +845,20 @@ template unsafeError*[T](self: Result[T, void]) =
   ## See also: `unsafeGet`
   assert not self.oResultPrivate # Emulate field access defect in debug builds
 
+func optValue*[T, E](self: Result[T, E]): Opt[T] =
+  ## Return the value of a Result as an Opt, or none if Result is an error
+  if self.oResultPrivate:
+    Opt.some(self.vResultPrivate)
+  else:
+    Opt.none(T)
+
+func optError*[T, E](self: Result[T, E]): Opt[E] =
+  ## Return the error of a Result as an Opt, or none if Result is a value
+  if self.oResultPrivate:
+    Opt.none(E)
+  else:
+    Opt.some(self.eResultPrivate)
+
 # Alternative spellings for get
 template value*[T, E](self: Result[T, E]): T = self.get()
 template value*[T: not void, E](self: var Result[T, E]): var T = self.get()

tests/test_results.nim

@@ -159,6 +159,12 @@ block:
   # Expectations
   doAssert rOk.expect("testOk never fails") == 42
 
+  # Conversions to Opt
+  doAssert rOk.optValue() == Opt.some(rOk.get())
+  doAssert rOk.optError().isNone()
+  doAssert rErr.optValue().isNone()
+  doAssert rErr.optError() == Opt.some(rErr.error())
+
   # Question mark operator
   func testQn(): Result[int, string] =
     let x = ?works() - ?works()
@@ -371,6 +377,10 @@ block: # Result[T, void] aka `Opt`
   doAssert Opt.some(42).get() == 42
   doAssert Opt.none(int).isNone()
 
+  # Construct Result from Opt
+  doAssert oOk.orErr("error").value() == oOk.get()
+  doAssert oErr.orErr("error").error() == "error"
+
 block: # `cstring` dangling reference protection
   type CSRes = Result[void, cstring]