graph/multi: fix issues

src/experimental/graph/multi.nim

@@ -37,9 +37,15 @@ method updateAt*(x: Gvalue, i: Gvalue, y: Gvalue): Gvalue {.base.} = raiseErrorB
 proc getAtmb(zb: Gvalue, z: Gvalue, i: int, dep: Gvalue): Gvalue =
   case i
   of 0:
-    return z.inputs[0].newOneOf.updateAt(i, zb)
+    if zb == nil:
+      # the output must be a scalar, otherwise crash later
+      return z.inputs[0].newOneOf.updateAt(z.inputs[1], toGvalue(1.0))
+    else:
+      return z.inputs[0].newOneOf.updateAt(z.inputs[1], zb)
+  of 1:
+    return toGvalue(0)
   else:
-    raiseValueError("i must be 0, got: " & $i)
+    raiseValueError("i must be 0 or 1, got: " & $i)
 
 proc getAtmf(v: Gvalue) =
   let x = Gmulti(v.inputs[0])
@@ -56,22 +62,27 @@ method `[]`*(x: Gmulti, i: Gint): Gvalue =
 proc updateAtmb(zb: Gvalue, z: Gvalue, i: int, dep: Gvalue): Gvalue =
   case i
   of 0:
-    return zb.updateAt(i, zb.inputs[0].newOneOf)
+    return zb.updateAt(z.inputs[1], z.inputs[2].newOneOf)
+  of 1:
+    return toGvalue(0)
   of 2:
     return zb[i]
   else:
-    raiseValueError("i must be 0 or 2, got: " & $i)
+    raiseValueError("i must be 0, 1, or 2, got: " & $i)
 
 proc updateAtmf(v: Gvalue) =
   let x = Gmulti(v.inputs[0])
   let i = v.inputs[1].getint
   let z = Gmulti(v)
   for k in 0..<z.mval.len:
     if k == i:
-      z.mval[k] = v.inputs[2]
+      z.mval[k].valCopy(v.inputs[2])
     else:
-      z.mval[k] = x.mval[i]
+      z.mval[k].valCopy(x.mval[k])
 
 let updateAtm = newGfunc(forward = updateAtmf, backward = updateAtmb, name = "updateAtm")
 
-method updateAt*(x: Gmulti, i: Gint, y: Gvalue): Gvalue = Gmulti(mval: newseq[Gvalue](x.mval.len), inputs: @[Gvalue(x), i, y], gfunc: updateAtm)
+method updateAt*(x: Gmulti, i: Gint, y: Gvalue): Gvalue =
+  result = x.newOneOf
+  result.inputs = @[Gvalue(x), i, y]
+  result.gfunc = updateAtm