results: add mapConvertErr, mapCastErr

We already have `mapConvert` and `mapCast` - this completes the API with
corresponding `Err` versions similar to `mapErr`.

The `Convert` / `Cast` operators are of somewhat dubious value - ie
they exist as "efficiency" shortcuts for `map` for the case that the
mapping should be done as a simple cast / conversion - an alternative
would be to deprecate these features and aim for some other, more
generic version that involves a type conversion library such as
#34, though this inherently,
and perhaps rightly, would be limited to "error-free" conversions.

Regardless, these helpers provide balance to the existing API.

stew/results.nim

@@ -603,6 +603,36 @@ func mapCast*[T0, E](
     else:
       result.err(self.eResultPrivate)
 
+func mapConvertErr*[T, E0](
+    self: Result[T, E0], E1: type): Result[T, E1] {.inline.} =
+  ## Convert result error to E1 using an conversion
+  # Would be nice if it was automatic...
+  when E0 is E1:
+    result = self
+  else:
+    if self.oResultPrivate:
+      when T is void:
+        result.ok()
+      else:
+        result.ok(self.vResultPrivate)
+    else:
+      when E1 is void:
+        result.err()
+      else:
+        result.err(E1(self.eResultPrivate))
+
+func mapCastErr*[T, E0](
+    self: Result[T, E0], E1: type): Result[T, E1] {.inline.} =
+  ## Convert result value to A using a cast
+  ## Would be nice with nicer syntax...
+  if self.oResultPrivate:
+    when T is void:
+      result.ok()
+    else:
+      result.ok(self.vResultPrivate)
+  else:
+    result.err(cast[E1](self.eResultPrivate))
+
 template `and`*[T0, E, T1](self: Result[T0, E], other: Result[T1, E]): Result[T1, E] =
   ## Evaluate `other` iff self.isOk, else return error
   ## fail-fast - will not evaluate other if a is an error

tests/test_results.nim

@@ -122,6 +122,9 @@ block:
   doAssert (rErr.orErr(32)).error == 32
   doAssert (rOk.orErr(failFast())).get() == rOk.get()
 
+  doAssert rErr.mapConvertErr(cstring).error() == cstring(rErr.error())
+  doAssert rErr.mapCastErr(cstring).error() == cast[cstring](rErr.error())
+
   # string conversion
   doAssert $rOk == "ok(42)"
   doAssert $rErr == "err(dummy)"