add Op:update_out and use in builtins

3 files changed, 27 insertions, 7 deletions

diff --git a/alv/base/op.moon b/alv/base/op.moon
index 982d75b..e751145 100644
--- a/alv/base/op.moon
+++ b/alv/base/op.moon
@@ -4,6 +4,7 @@
 -- @classmod Op
 import deep_copy, deep_iter, deep_map from require 'alv.util'
 import T from require 'alv.type'
+import SigStream, EvtStream from require 'alv.result'
 
 class Op
 --- members
@@ -160,6 +161,26 @@ class Op
       @inputs = inputs
       do_setup old_inputs, @inputs
 
+  --- create or update `out`.
+  --
+  -- This should be used instead of setting `out` directly.
+  -- It will try to keep the current value if possible.
+  --
+  -- @tparam string metatype (one of `!` or `~`)
+  -- @tparam Type type
+  -- @tparam[opt] any val initial value
+  update_out: (metatype, type, val) =>
+    same_type = @out and @out.type == type
+    if same_type and @out.metatype == metatype
+      -- we can just keep it. do nothing.
+      return
+
+    -- prefer last value if applicable
+    val = same_type and @.out! or val
+
+    ResultType = if metatype == '!' then EvtStream else SigStream
+    @out = ResultType type, val
+
   --- `\unwrap` all `Input`s in `@inputs` and return a table with the same
   -- shape.
   --
diff --git a/alv/base/pureop.moon b/alv/base/pureop.moon
index 10c737c..449519a 100644
--- a/alv/base/pureop.moon
+++ b/alv/base/pureop.moon
@@ -56,7 +56,8 @@ class PureOp extends Op
     typ = if (type @type) == 'function' then @type args else @type
     if typ
       assert (ancestor typ.__class) == Type, "not a type: #{typ}"
-      @out = if trigger then typ\mk_evt! else typ\mk_sig!
+      metatype = if trigger then '!' else '~'
+      @update_out metatype, typ
 
     map_fn = if trigger then hot_if_trigger trigger else Input.hot
     inputs = deep_map args, map_fn
diff --git a/alv/builtins.moon b/alv/builtins.moon
index bf3c95f..a015bc6 100644
--- a/alv/builtins.moon
+++ b/alv/builtins.moon
@@ -337,10 +337,8 @@ switch_ = Constant.meta
     setup: (inputs) =>
       { i, values } = pattern\match inputs
 
-      @out = if values[1].result.metatype ~= '!'
-        values[1]\type!\mk_sig!
-      else
-        values[1]\type!\mk_evt!
+      val1 = values[1]
+      @update_out val1.result.metatype, val1.result.type
 
       if i\type! == T.bang
         @state or= 1
@@ -463,8 +461,7 @@ Since ~-streams cannot be emtpy, specifying an `initial` value is necessary."
 
       super event: Input.hot event
 
-      if not @out or @out.type != event\type!
-        @out = event\type!\mk_sig initial.result!
+      @update_out '~', event\type!, initial.result!
 
     tick: => @out\set @inputs.event!
 
@@ -493,6 +490,7 @@ to_evt = Constant.meta
           sig: Input.cold Constant.bang true
       else
         super sig: Input.hot sig_
+
       @out = @inputs.sig\type!\mk_evt!
 
     tick: (setup) =>