direnv and environment variables

Question

I'm trying to get autoactivation of conda/mamba environments working using direnv but I'm running across a problem that seems to be due to my lack of understanding of how bash scripts work with environment variables.

My shell is fish but direnv is bash based. I don't think my shell being fish has any bearing on the problem since I run the appropriate direnv hook for fish.

My .direnvrc file is as follows:

layout_mamba() {
  local MAMBA_EXE="${HOME}/bin/micromamba" # Make sure this points to path of your micromamba executable
  # initialize micromamba
  eval "$(${MAMBA_EXE} shell hook --shell=bash)"
  if [ -n "$1" ]; then
    # Explicit environment name from layout command.
    local env_name="$1"
    micromamba activate ${env_name}
  elif (grep -q name: environment.yml); then
    # Detect environment name from `environment.yml` file in `.envrc` directory
    micromamba activate `grep name: environment.yml | sed -e 's/name: //'`
  else
    (>&2 echo No environment specified);
    exit 1;
  fi;
}

And it works as is but notice that in my eval statement, I use the value of $MAMBA_EXE but the remaining calls micromamba directly. Everything that I've tried for substituting $MAMBA_EXE for micromamba has failed.

Any suggestions for making my .direnvrc more general?

Edit: For example, if I change line 8 from micromamba activate ${env_name} to ${MAMBA_EXE} activate ${env_name} I get:

direnv: loading ~/test-env/.envrc

'micromamba' is running as a subprocess and can't modify the parent shell.
Thus you must initialize your shell before using activate and deactivate.

    To initialize the current bash shell, run:
        $ eval "$(micromamba shell hook --shell=bash)"
    and then activate or deactivate with:
        $ micromamba activate
    
To automatically initialize all future (bash) shells, run:
    $ micromamba shell init --shell=bash --prefix=~/micromamba

Supported shells are {bash, zsh, csh, xonsh, cmd.exe, powershell, fish}.

critical libmamba Shell not initialized

Fish won't let me run ${MAMBA_EXE} shell hook --shell=bash but running $MAMBA_EXE shell hook --shell=bash produces:

# Copyright (C) 2012 Anaconda, Inc
# SPDX-License-Identifier: BSD-3-Clause

__mamba_exe() (
    "$MAMBA_EXE" "$@"
)

__mamba_hashr() {
    if [ -n "${ZSH_VERSION:+x}" ]; then
        \rehash
    elif [ -n "${POSH_VERSION:+x}" ]; then
        :  # pass
    else
        \hash -r
    fi
}

__mamba_activate() {
    \local ask_conda
    ask_conda="$(PS1="${PS1:-}" __mamba_exe shell --shell bash "$@")" || \return
    \eval "$ask_conda"
    __mamba_hashr
}

__mamba_reactivate() {
    \local ask_conda
    ask_conda="$(PS1="${PS1:-}" __mamba_exe shell --shell bash reactivate)" || \return
    \eval "$ask_conda"
    __mamba_hashr
}

micromamba() {
    \local cmd="${1-__missing__}"
    case "$cmd" in
        activate|deactivate)
            __mamba_activate "$@"
            ;;
        install|update|upgrade|remove|uninstall)
            __mamba_exe "$@" || \return
            __mamba_reactivate
            ;;
        self-update)
            __mamba_exe "$@" || \return

            # remove leftover backup file on Windows
            if [ -f "$MAMBA_EXE.bkup" ]; then
                rm -f $MAMBA_EXE.bkup
            fi
            ;;
        *)
            __mamba_exe "$@"
            ;;
    esac
}

if [ -z "${CONDA_SHLVL+x}" ]; then
    \export CONDA_SHLVL=0
    # In dev-mode MAMBA_EXE is python.exe and on Windows
    # it is in a different relative location to condabin.
    if [ -n "${_CE_CONDA+x}" ] && [ -n "${WINDIR+x}" ]; then
        PATH="${MAMBA_ROOT_PREFIX}/condabin:${PATH}"
    else
        PATH="${MAMBA_ROOT_PREFIX}/condabin:${PATH}"
    fi
    \export PATH

    # We're not allowing PS1 to be unbound. It must at least be set.
    # However, we're not exporting it, which can cause problems when starting a second shell
    # via a first shell (i.e. starting zsh from bash).
    if [ -z "${PS1+x}" ]; then
        PS1=
    fi
fi

if [ -n "${ZSH_VERSION:+x}" ]; then
  if ! command -v compinit > /dev/null; then
    autoload -U +X compinit && if [[ "${ZSH_DISABLE_COMPFIX-}" = true ]]; then
      compinit -u
    else
      compinit
    fi
  fi
  autoload -U +X bashcompinit && bashcompinit

  _umamba_zsh_completions()
  {
    COMPREPLY=($(__mamba_exe completer "${(@s: :)${(@s: :)COMP_LINE}:1}"))
  }

  complete -o default -F _umamba_zsh_completions micromamba
fi
if [ -n "${BASH_VERSION:+x}" ]; then
  _umamba_bash_completions()
  {
    COMPREPLY=($(__mamba_exe completer "${COMP_WORDS[@]:1}"))
  }
  complete -o default -F _umamba_bash_completions micromamba
fi