Convert json to svg by a tool named bit-field

bit-field.mk

SRCS := $(wildcard *.json)
OBJS := $(patsubst %.json,%.svg, $(SRCS))

all: $(OBJS)

$(OBJS): %.svg: %.json
	npx bit-field --hspace 1800 --lanes $(shell echo $< | grep -o -E '[0-9]*') --vflip --hflip -i $< > $@

clean:
	rm -f $(OBJS)

.PHONY: all clean

At first, I'd like to define a function variable like this

get_lanes := $(shell echo $(1) | grep -o -E '[0-9]*')

and the command in the rule will looks like

npx bit-field --hspace 1800 --lanes $(call get_lanes, $<) --vflip --hflip -i $< > $@

Unfortunately, it doesn't work.

the problem is :=. the main differenece between := and = lies in when the expansion happens.

immediate = deferred
immediate ?= deferred
immediate := immediate
immediate ::= immediate
immediate :::= immediate-with-escape
immediate += deferred or immediate
immediate != immediate