Merge branch 'develop-spec-arg'. Close #446.

**Description**

The type `Copilot.Language.Stream.Arg` is defined in the same module as
a `Stream`, but nothing in that module uses an `Arg`. Architecturally,
that type doesn't belong in that module, but, rather, should be listed
in `Copilot.Language.Spec`, where it is used in the definition of
`Trigger`.

Consequently, `Copilot.Language.Stream.Arg` should be moved to
`Copilot.Language.Spec`.

As per our internal policy of waiting 3 versions from deprecation until
a public interface declaration can be hidden, this definition should be
deprecated in `Copilot.Language.Stream.Arg` and re-published from
`Copilot.Language.Spec`. However, Haskell offers no mechanism to do
that. If we deprecate it in `Copilot.Language.Stream.Arg`, we cannot
make that warning disappear when imported from `Copilot.Language.Spec`,
even with `{-# OPTIONS_GHC -fno-warn-deprecations #-}`.

**Type**

- Bug: low cohesion in `Copilot.Language.Stream`.

**Additional context**

None.

**Requester**

- Ivan Perez

**Method to check presence of bug**

This is a bug in architecture and design. There is no way to check for
this bug automatically.

There are signs of this bug. For example, running a search on the
codebase indicates that nothing inside `Stream` uses `Arg`, but that is
not totally uncommon in modules that define datatypes.

In this case, it is important to understand that, conceptually, an `Arg`
only makes sense in the context of a `Trigger`, so they belong together.
Because `Trigger` is defined in `Copilot.Language.Spec`, then so should
`Arg`.

We can, however, check that it is defined in `Spec`, as follows:

```Dockerfile
FROM ubuntu:trusty

RUN apt-get update

RUN apt-get install --yes software-properties-common
RUN add-apt-repository ppa:hvr/ghc
RUN apt-get update

RUN apt-get install --yes ghc-8.6.5 cabal-install-2.4
RUN apt-get install --yes libz-dev

ENV PATH=/opt/ghc/8.6.5/bin:/opt/cabal/2.4/bin:$PWD/.cabal-sandbox/bin:$PATH

RUN cabal update
RUN cabal v1-sandbox init
RUN cabal v1-install alex happy
RUN apt-get install --yes git

SHELL ["/bin/bash", "-c"]
CMD git clone $REPO && cd $NAME && git checkout $COMMIT && cd .. \
  && cabal v1-install copilot/copilot-**/  \
  && ! cabal v1-exec -- runhaskell <<< "import Copilot.Language.Stream (Arg); main :: IO (); main = return ()" \
  && cabal v1-exec -- runhaskell <<< "import Copilot.Language.Spec (Arg(Arg)); main :: IO (); main = return ()" \
  && echo "Success"
```

Command (substitute variables based on new path after merge):
```
$ docker run -e "REPO=https://github.com/Copilot-Language/copilot" -e "NAME=copilot" -e "COMMIT=<HASH>" -it copilot-verify-446
```

**Expected result**

Running the script above prints Success, indicating that `Arg` is
defined in and exported from `Copilot.Language.Spec`, and not
`Copilot.Language.Stream`.

**Solution implemented**

Move `Arg` from `Copilot.Language.Stream` to `Copilot.Language.Spec`.

Adjust imports accordingly.

**Further notes**

We provide the function `arg` precisely so that people do not use the
constructor `Arg` directly.

Author: ivanperez-keera

copilot-language/CHANGELOG

@@ -1,3 +1,6 @@
+2023-07-07
+        * Move Copilot.Language.Stream.Arg to Copilot.Language.Spec. (#446)
+
 2023-05-07
         * Version bump (3.15). (#438)
         * Remove outdated comment about pretty-printer. (#428)

copilot-language/src/Copilot/Language/Analyze.hs

@@ -14,7 +14,7 @@ module Copilot.Language.Analyze
 
 import Copilot.Core (DropIdx)
 import qualified Copilot.Core as C
-import Copilot.Language.Stream (Stream (..), Arg (..))
+import Copilot.Language.Stream (Stream (..))
 import Copilot.Language.Spec
 import Copilot.Language.Error (badUsage)
 

copilot-language/src/Copilot/Language/Reify.hs

@@ -17,7 +17,7 @@ import Copilot.Core (Typed, Id, typeOf)
 import Copilot.Language.Analyze (analyze)
 import Copilot.Language.Error   (impossible)
 import Copilot.Language.Spec
-import Copilot.Language.Stream (Stream (..), Arg (..))
+import Copilot.Language.Stream (Stream (..))
 
 import Copilot.Theorem.Prove
 

copilot-language/src/Copilot/Language/Spec.hs

@@ -22,6 +22,7 @@ module Copilot.Language.Spec
   , Trigger (..)
   , trigger, triggers
   , arg
+  , Arg(..)
   , Property (..)
   , Prop (..)
   , prop, properties
@@ -198,3 +199,7 @@ theorem name e (Proof p) = tell [TheoremItem (Property name (extractProp e), p)]
 -- the current samples of the given streams.
 arg :: Typed a => Stream a -> Arg
 arg = Arg
+
+-- | Wrapper to use 'Stream's as arguments to triggers.
+data Arg where
+  Arg :: Typed a => Stream a -> Arg

copilot-language/src/Copilot/Language/Stream.hs

@@ -9,7 +9,6 @@
 
 module Copilot.Language.Stream
   ( Stream (..)
-  , Arg (..)
   , Copilot.Language.Stream.ceiling
   , Copilot.Language.Stream.floor
   , Copilot.Language.Stream.atan2
@@ -48,10 +47,6 @@ data Stream :: * -> * where
               => Core.Op3 a b c d -> Stream a -> Stream b -> Stream c -> Stream d
   Label       :: Typed a => String -> Stream a -> Stream a
 
--- | Wrapper to use 'Stream's as arguments to triggers.
-data Arg where
-  Arg :: Typed a => Stream a -> Arg
-
 -- | Dummy instance in order to make 'Stream' an instance of 'Num'.
 instance Show (Stream a) where
   show _      = "Stream"