copilot-language: Add type annotation to help type inference engine. Refs #469.

[ivanperez-keera]

Add type signature to an internal definition that uses values of an existential type, making the code for the tests compile with newer versions of GHC (>= 9.4), as required by #469 .

[ivanperez-keera]

Change Manager: Verified that:

• Solution is implemented:
  • The code proposed compiles and passes all tests. Details:
    Build log: https://app.travis-ci.com/github/Copilot-Language/copilot/builds/267903584
  • The solution proposed produces the expected result. Details:
    The following Dockerfile checks that copilot-language compiles with GHC 9.4, in which case it prints the message Success:

    FROM ubuntu:focal
    
    RUN apt-get update
    
    RUN apt-get install --yes libz-dev
    RUN apt-get install --yes git
    
    RUN apt-get install --yes wget
    RUN mkdir -p $HOME/.ghcup/bin
    RUN wget https://downloads.haskell.org/~ghcup/0.1.19.2/x86_64-linux-ghcup-0.1.19.2 -O $HOME/.ghcup/bin/ghcup
    
    RUN chmod a+x $HOME/.ghcup/bin/ghcup
    ENV PATH=$PATH:/root/.ghcup/bin/
    ENV PATH=$PATH:/root/.cabal/bin/
    RUN apt-get install --yes curl
    RUN apt-get install --yes gcc make libgmp3-dev g++
    
    SHELL ["/bin/bash", "-c"]
    
    RUN ghcup install ghc 9.4.7
    RUN ghcup install cabal 3.8
    RUN ghcup set ghc 9.4.7
    RUN cabal update
    
    CMD git clone $REPO && \
        cd $NAME && \
        git checkout $COMMIT && \
        cd copilot-language && \
        cabal test . && \
        echo Success

    Command (substitute variables based on new path after merge):

    docker run -e "REPO=https://github.com/ivanperez-keera/copilot" -e "NAME=copilot" -e "COMMIT=e25c7afaf743a5fdc7a9638516f59b006383bef0" -it copilot-verify-469
    

• Implementation is documented. Details:
  Internal change; no changes to the documentation are needed, and a comment is added to ensure that future revisions do not remove the change accidentally.
• Change history is clear.
• Commit messages are clear.
• Changelogs are updated.
• Examples are updated. Details:
  No updates needed.
• Required version bumps are evaluated. Details:
  Bump not needed (internal change to tests; does not affect API).