devenv: Only define targets when tool is not available

An issue with the previous approach is that if the target exists and
happened to be older than the tool used to bootstrap it, `make` would
try to re-install it!

Instead when falling back to an existing install we don't define a
target at all, sidestepping the issue.

This commit also introduces some `define` rules allowing us to write a
template rule once and initialize it multiple times with different
parameters - useful for defining multiple versions of Python!

.devcontainer/tools.mk

@@ -1,91 +1,105 @@
 BIN ?= $(HOME)/.local/bin
 
-PY38 ?= $(or $(shell command -v python3.8), $(BIN)/python3.8)
-PY39 ?= $(or $(shell command -v python3.9), $(BIN)/python3.9)
-PY310 ?= $(or $(shell command -v python3.10), $(BIN)/python3.10)
-PY311 ?= $(or $(shell command -v python3.11), $(BIN)/python3.11)
-PY312 ?= $(or $(shell command -v python3.12), $(BIN)/python3.12)
+HATCH_VERSION = 1.10.0
+NODE_VERSION := 18.20.2
 
-# Set a default python
-PY ?= $(or $(shell command -v python), $(BIN)/python)
-PY_INTERPRETERS := $(PY) $(PY38) $(PY39) $(PY310) $(PY311) $(PY312)
+# The versions of Python we support
+PYXX_versions := 3.8 3.9 3.10 3.11 3.12
+PY_INTERPRETERS =
 
+# Hatch is not only used for building packages, but bootstrapping any missing
+# interpreters
 HATCH ?= $(or $(shell command -v hatch), $(BIN)/hatch)
-HATCH_VERSION = 1.9.7
-PRE_COMMIT ?= $(or $(shell command -v pre-commit), $(BIN)/pre-commit)
-TOWNCRIER ?= $(or $(shell command -v towncrier), $(BIN)/towncrier)
-
-PY_TOOLS := $(HATCH) $(PRE_COMMIT) $(TOWNCRIER)
-
-# Node JS
-NPM ?= $(or $(shell command -v npm), $(BIN)/npm)
-NODE ?= $(or $(shell command -v node), $(BIN)/node)
-NODE_VERSION := 18.20.2
-NODE_DIR := $(HOME)/.local/node
-
-tools: $(PY_INTERPRETERS) $(PY_TOOLS) $(NPM) $(NODE)
-	for prog in $^ ; do echo -n "$${prog}\t" ; $${prog} --version; done
 
 $(HATCH):
 	curl -L --output /tmp/hatch.tar.gz https://github.com/pypa/hatch/releases/download/hatch-v$(HATCH_VERSION)/hatch-$(HATCH_VERSION)-x86_64-unknown-linux-gnu.tar.gz
 	tar -xf /tmp/hatch.tar.gz -C /tmp
 	rm /tmp/hatch.tar.gz
 
-	[ -d $(BIN) ] || mkdir -p $(BIN)
+	test -d $(BIN) || mkdir -p $(BIN)
 	mv /tmp/hatch-$(HATCH_VERSION)-x86_64-unknown-linux-gnu $(HATCH)
 
 	$@ --version
 	touch $@
 
-$(PY38): $(HATCH)
-	$(HATCH) python find 3.8  || $(HATCH) python install 3.8
-	ln -s $$($(HATCH) python find 3.8) $@
+# This effectively defines a function `PYXX` that takes a Python version number
+# (e.g. 3.8) and expands it out into a common block of code that will ensure a
+# verison of that interpreter is available to be used.
+#
+# The is perhaps a bit more complicated than I'd like, but it should mean that
+# the project's makefiles are useful both inside and outside of a devcontainer.
+#
+# `PYXX` has the following behavior:
+# - If possible, it will reuse the user's existing version of Python
+#   i.e. $(shell command -v pythonX.X)
+#
+# - The user may force a specific interpreter to be used by setting the
+#   variable when running make e.g. PYXX=/path/to/pythonX.X make ...
+#
+# - Otherwise, `make` will use `$(HATCH)` to install the given version of
+#   Python under `$(BIN)`
+#
+# See: https://www.gnu.org/software/make/manual/html_node/Eval-Function.html
+define PYXX =
 
-	$@ --version
-	touch $@
+PY$(subst .,,$1) ?= $$(shell command -v python$1)
 
-$(PY39): $(HATCH)
-	$(HATCH) python find 3.9  || $(HATCH) python install 3.9
-	ln -s $$($(HATCH) python find 3.9) $@
+ifeq ($$(strip $$(PY$(subst .,,$1))),)
 
-	$@ --version
-	touch $@
+PY$(subst .,,$1) := $$(BIN)/python$1
 
-$(PY310): $(HATCH)
-	$(HATCH) python find 3.10  || $(HATCH) python install 3.10
-	ln -s $$($(HATCH) python find 3.10) $@
+$$(PY$(subst .,,$1)): $$(HATCH)
+	$$(HATCH) python find $1 || $$(HATCH) python install $1
+	ln -s $$$$($$(HATCH) python find $1) $$@
 
-	$@ --version
-	touch $@
+	$$@ --version
+	touch $$@
 
-$(PY311): $(HATCH)
-	$(HATCH) python find 3.11  || $(HATCH) python install 3.11
-	ln -s $$($(HATCH) python find 3.11) $@
+endif
 
-	$@ --version
-	touch $@
+PY_INTERPRETERS += $$(PY$(subst .,,$1))
+endef
 
-$(PY312): $(HATCH)
-	$(HATCH) python find 3.12  || $(HATCH) python install 3.12
-	ln -s $$($(HATCH) python find 3.12) $@
+# Uncomment the following line to see what this expands into.
+#$(foreach version,$(PYXX_versions),$(info $(call PYXX,$(version))))
+$(foreach version,$(PYXX_versions),$(eval $(call PYXX,$(version))))
 
-	$@ --version
-	touch $@
+# Set a default `python` command if there is not one already
+PY ?= $(shell command -v python)
+
+ifeq ($(strip $(PY)),)
+PY := $(BIN)/python
 
 $(PY): $(PY312)
 	ln -s $< $@
 	$@ --version
 	touch $@
+endif
+
+PY_INTERPRETERS += $(PY)
+#$(info $(PY_INTERPRETERS))
+
+PRE_COMMIT ?= $(shell command -v pre-commit)
+
+ifeq ($(strip $(PRE_COMMIT)),)
+PRE_COMMIT := $(BIN)/pre-commit
 
 $(PRE_COMMIT): $(PY)
 	$(PY) -m pip install --user pre-commit
 	$@ --version
 	touch $@
+endif
 
-$(TOWNCRIER): $(PY)
-	$(PY) -m pip install --user towncrier
-	$@ --version
-	touch $@
+PY_TOOLS := $(HATCH) $(PRE_COMMIT)
+
+# Node JS
+NPM ?= $(shell command -v npm)
+
+ifeq ($(strip $(NPM)),)
+
+NPM := $(BIN)/npm
+NODE := $(BIN)/node
+NODE_DIR := $(HOME)/.local/node
 
 $(NPM):
 	curl -L --output /tmp/node.tar.xz https://nodejs.org/dist/v$(NODE_VERSION)/node-v$(NODE_VERSION)-linux-x64.tar.xz
@@ -101,3 +115,9 @@ $(NPM):
 
 	$(NODE) --version
 	PATH=$(BIN) $(NPM) --version
+
+endif
+
+# One command to bootstrap all tools and check their versions
+tools: $(PY_INTERPRETERS) $(PY_TOOLS) $(NPM)
+	for prog in $^ ; do echo -n "$${prog}\t" ; PATH=$(BIN) $${prog} --version; done