Makefile does not find target - makefile

Question

I have the following Makefile, but it does not work. When I call
make html
I get a
make: *** No rule to make target `docs/index.html', needed by `html'. Stop.
error, even though I think I have defined it.
SRCDIR = source
OUTDIR = docs
RMD = $(wildcard $(SRCDIR)/*.Rmd)
TMP = $(RMD:.Rmd=.html)
HTML = ${subst $(SRCDIR),$(OUTDIR),$(TMP)}
test:
echo $(RMD)
echo $(TMP)
echo $(HTML)
all: clean update html
html: $(HTML)
%.html: %.Rmd
echo $(HTML)
#Rscript -e "rmarkdown::render('$<', output_format = 'prettydoc::html_pretty', output_dir = './$(OUTDIR)/')"
update:
#Rscript -e "devtools::load_all(here::here()); microcosmScheme:::updateFromGoogleSheet(token = './source/googlesheets_token.rds')"
## from https://stackoverflow.com/a/26339924/632423
list:
#$(MAKE) -pRrq -f $(lastword $(MAKEFILE_LIST)) : 2>/dev/null | awk -v RS= -F: '/^# File/,/^# Finished Make data base/ {if ($$1 !~ "^[#.]") {print $$1}}' | sort | egrep -v -e '^[^[:alnum:]]' -e '^$#$$' | xargs
.PHONY: update clean cleanhtml all list
The variables seem to be correct:
15:21 $ make test
echo source/index.Rmd
source/index.Rmd
echo source/index.html
source/index.html
echo docs/index.html
docs/index.html
If I change it as follow it works, but the target points to the SRCDIR, but I want it to point to the OUTDIR:
RMD = $(wildcard $(SRCDIR)/*.Rmd)
HTML = $(RMD:.Rmd=.html)
# HTML = ${subst $(SRCDIR),$(OUTDIR),$(TMP)}
I am sure it is one small thing...

Answer 1

This rule:
%.html : %.Rmd
....
tells make how to build a file foo.html from a file foo.Rmd, or a file source/foo.html from a file source/foo.Rmd, or a file docs/foo.html from a file docs/foo.Rmd.
It doesn't tell make how to build a file docs/foo.html from a file source/foo.Rmd, because the stem that matches the pattern % is not the same.
If you want to write a pattern for docs/foo.html to be built from source/foo.Rmd, you have to write it like this:
$(OUTDIR)/%.html : $(SRCDIR)/%.Rmd
....
so that the part that matches the pattern % is identical.
ETA Some other notes: you should be using := with the wildcard function as it's much better performing. Also you shouldn't use subst here because it replaces all occurrences of the string which could break things if any of your .Rmd files contain the string source for example (e.g., source/my_source_file.Rmd. This is much better written with patsubst, as in:
RMD := $(wildcard $(SRCDIR)/*.Rmd)
HTML := $(patsubst $(SRCDIR)/%.Rmd,$(OBJDIR)/%.html,$(RMD))
Finally, you don't show what the clean target does but it's unusual to have the clean target depended on by all. Usually it's a separate target that is invoked only when you want it, like make clean.