heterogenous `or`

Useful for translating `error` results

stew/results.nim

@@ -373,15 +373,29 @@ template `and`*[T0, E, T1](self: Result[T0, E], other: Result[T1, E]): Result[T1
   if s.o:
     other
   else:
-    type R = type(other)
-    R.err(s.e)
+    when type(self) is type(other):
+      s
+    else:
+      type R = type(other)
+      err(R, s.e)
 
-template `or`*[T, E](self, other: Result[T, E]): Result[T, E] =
-  ## Evaluate `other` iff not self.isOk, else return self
-  ## fail-fast - will not evaluate other if a is a value
+template `or`*[T, E0, E1](self: Result[T, E0], other: Result[T, E1]): Result[T, E1] =
+  ## Evaluate `other` iff `not self.isOk`, else return `self`
+  ## fail-fast - will not evaluate `other` if `self` is ok
+  ##
+  ## ```
+  ## func f(): Result[int, SomeEnum] =
+  ##   f2() or err(EnumValue) # Collapse errors from other module / function
+  ## ```
   let s = self
-  if s.o: s
-  else: other
+  if s.o:
+    when type(self) is type(other):
+      s
+    else:
+      type R = type(other)
+      ok(R, s.v)
+  else:
+    other
 
 template catch*(body: typed): Result[type(body), ref CatchableError] =
   ## Catch exceptions for body and store them in the Result

tests/test_results.nim

@@ -35,6 +35,9 @@ doAssert (rErr or rOk).isOk
 # `and` heterogenous types
 doAssert (rOk and rOk.map(proc(x: auto): auto = $x))[] == $(rOk[])
 
+# `or` heterogenous types
+doAssert (rErr or rErr.mapErr(proc(x: auto): auto = len(x))).error == len(rErr.error)
+
 # Exception on access
 let va = try: discard rOk.error; false except: true
 doAssert va, "not an error, should raise"
@@ -182,6 +185,13 @@ doAssert testQn()[] == 0
 doAssert testQn2().isErr
 doAssert testQn3()[]
 
+proc heterOr(): Result[int, int] =
+  let value = ?(rErr or err(42))  # TODO ? binds more tightly than `or` - can that be fixed?
+  doAssert value + 1 == value, "won't reach, ? will shortcut execution"
+  ok(value)
+
+doAssert heterOr().error() == 42
+
 type
   AnEnum = enum
     anEnumA