Move ghcup to XDG dirs

Addresses #17
 - Adds `XDG_BIN_HOME` environment variable for user's binaries and adds
   it to appropriate places in Makefile
 - Introduces `GHCUP_USE_XDG_DIRS` to enable `gucp`'s XDG support, see:
   https://www.haskell.org/ghcup/guide/#xdg-support
 - Updates path for sourcing `ghcup` env in `.zshrc`
 - Adds potential deprecation note for `make stack`

This change requires reinstall, however it does not seem to break
existing stack projects. Migration can look as follows:
 1. `~/.ghcup`, `~/.cabal` and `~/.ghc` can be dropped
 2. Reload shell to bring `GHCUP_USE_XDG_DIRS` in scope
 3. Reinstall with `make ghcup` with HLS and `stack` as well - `.zshrc`
    update is not necessary as it's part of this commit
 4. Reload shell again to source ghcup's env

Known issues:
 - Shell completions for `ghcup` don't seem to work (could have been a
   pre-existing issue)
 - Stack must now be updated via `ghcup` (according to a post-install
   log message)

.zshenv

@@ -14,6 +14,11 @@
 #  - Furthermore, `.zprofile` might be better place for commands and variables
 #    that should be set *once* or which **don't need to be updated frequently** 
 
+# XDG specification does not define this variable but it is mentioned/assumed by
+# some apps/libs (e.g. `ghcup`) so it might be useful to export it anyway.
+#  - https://www.haskell.org/ghcup/guide/#xdg-support
+export XDG_BIN_HOME="${XDG_BIN_HOME:-$HOME/.local/bin}"
+
 # System and architecture
 export ARCH=$(uname -m)
 
@@ -48,6 +53,11 @@ export RUSTFLAGS="-C target-cpu=native"
 export RUST_BACKTRACE=1
 
 # Haskell stack
+#  - Any value of `GHCUP_USE_XDG_DIRS` will enable XDG support, see:
+#    https://www.haskell.org/ghcup/guide/#xdg-support
+#  - Note: `XDG_BIN_HOME` will be used for storing binaries and as mentioned in
+#    the docs, there might be clash with other tools using it (stack/cabal/ghc) 
+export GHCUP_USE_XDG_DIRS=1
 export STACK_ROOT=${XDG_DATA_HOME}/stack
 
 # Go lang

.zshrc

@@ -153,7 +153,7 @@ fi
 [ ! $(command -v fuck) ] || eval $(thefuck --alias)
 
 # Haskell toolchain installer ghcup (https://www.haskell.org/ghcup/)
-[ -f "/home/matyama/.ghcup/env" ] && source "/home/matyama/.ghcup/env"
+[ -f "${XDG_DATA_HOME}/ghcup/env" ] && source "${XDG_DATA_HOME}/ghcup/env"
 
 # Haskell stack (https://docs.haskellstack.org/en/stable/README/)
 [ ! $(command -v stack) ] || eval "$(stack --bash-completion-script stack)"

Makefile

@@ -9,6 +9,10 @@ ifndef XDG_CONFIG_HOME
 XDG_CONFIG_HOME=$(HOME)/.config
 endif
 
+ifndef XDG_BIN_HOME
+XDG_BIN_HOME=$(HOME)/.local/bin
+endif
+
 ifndef XDG_DATA_HOME
 XDG_DATA_HOME=$(HOME)/.local/share
 endif
@@ -61,6 +65,10 @@ ifndef TRAVIS_CONFIG_PATH
 TRAVIS_CONFIG_PATH=$(XDG_CONFIG_HOME)/travis
 endif
 
+ifndef GHCUP_USE_XDG_DIRS
+GHCUP_USE_XDG_DIRS=1
+endif
+
 ifndef STACK_ROOT
 STACK_ROOT=$(XDG_DATA_HOME)/stack
 endif
@@ -97,7 +105,7 @@ $(FONTS_DIR) \
 	$(ZSH_CUSTOM) \
 	$(ZSH)/completions \
 	$(ZSH)/plugins/poetry \
-	~/.local/bin \
+	$(XDG_BIN_HOME) \
 	$(XDG_CONFIG_HOME)/coc \
 	$(XDG_CONFIG_HOME)/direnv \
 	$(XDG_CONFIG_HOME)/git \
@@ -186,6 +194,7 @@ links: \
 	$(FD_CONFIG_HOME) \
 	$(RIPGREP_CONFIG_HOME) \
 	$(STACK_ROOT) \
+	$(XDG_BIN_HOME) \
 	$(XDG_CONFIG_HOME)/coc \
 	$(XDG_CONFIG_HOME)/direnv \
 	$(XDG_CONFIG_HOME)/git \
@@ -194,7 +203,6 @@ links: \
 	$(XDG_CONFIG_HOME)/nvim/scripts \
 	$(XDG_CONFIG_HOME)/pypoetry \
 	$(XDG_CONFIG_HOME)/zsh \
-	~/.local/bin \
 	$(ZSH_CUSTOM)
 	# END DEPS
 	@echo "Linking configuration files:"
@@ -207,7 +215,7 @@ links: \
 			ln -svf $$cfg "$(HOME)$${cfg#$(CFG_DIR)}";\
 		done;\
 	}
-	@ln -svft ~/.local/bin \
+	@ln -svft $(XDG_BIN_HOME) \
 		$(CFG_DIR)/.local/bin/alacritty_toggle.sh \
 		$(CFG_DIR)/.local/bin/increase_swap.sh \
 		$(CFG_DIR)/.local/bin/init_ubuntu.sh \
@@ -628,6 +636,9 @@ else
 	@echo ">>> Finish $@ completion setup by reloading zsh with 'zshreload'"
 endif
 
+# TODO: ghcup now installs stack as well => deprecate?
+#  - From `ghcup`'s post-installation log:
+#    "Additionally, you should upgrade stack only through ghcup."
 .PHONY: stack
 stack: net-tools
 ifneq ($(shell which stack 2> /dev/null),)
@@ -876,7 +887,7 @@ aws-vault: net-tools
 	sudo chmod 755 $(AWS_VAULT_BIN)
 
 .PHONY: bat
-bat: ~/.local/bin
+bat: $(XDG_BIN_HOME)
 ifneq ($(shell which bat 2> /dev/null),)
 	@echo ">>> $$($@ --version) already installed"
 else