code management: generate source files with slight variations of various rules

Question

I have a source file in a declarative language (twolc, actually) that I need to write many variations on: a normative version and many non-normative versions, each with one or more variations from the norm. For example, say the normative file has three rules:

Rule A:
    Do something A-ish

Rule B:
    Do something B-ish

Rule C:
    Do something C-ish

Then one variation might have the exact same rules as the norm for A and C, but a different rule for B, which I will call B-1:

Rule A:
    Do something A-ish

Rule B-1:
    Do something B-ish, but with a flourish

Rule C:
    Do something C-ish

Imagine that you have many different subtle variations on many different rules, and you have my situation. The problem I am worried about is code maintainability. If, later on, I decide that Rule A needs to be refactored somehow, then I will have 50+ files that need to have the exact same rule edited by hand.

My idea is to have separate files for each rule and concatenate them into variations using cat: cat A.twolc B.twolc C.twolc > norm.twolc, cat A.twolc B-1.twolc C.twolc > not-norm.twolc, etc.

Are there any tools designed to manage this kind of problem? Is there a better approach than the one I have in mind? Does my proposed solution have weaknesses I should watch out for?

I have tried searching the web, but this seems to be one of those times that not knowing the right terms is a blocker. — reynoldsnlp

Renaud Pacalet Renaud Pacalet · Accepted Answer · 2017-06-21T11:15:18

As you added the makefile tag, here is a GNU-make-based (and Gnu make only) solution:

# Edit this
RULES       := A B B-1 C
VARIATIONS  := norm not-norm
norm-rules  := A B C
not-norm-rules  := A B-1 C
# Do not edit below this line

VARIATIONSTWOLC := $(patsubst %,%.twolc,$(VARIATIONS))

all: $(VARIATIONSTWOLC)

define GEN_rules
$(1).twolc: $$(patsubst %,%.twolc,$$($(1)-rules))
    cat $$^ > $$@
endef
$(foreach v,$(VARIATIONS),$(eval $(call GEN_rules,$(v))))

clean:
    rm -f $(VARIATIONSTWOLC)

patsubst is straightforward. The foreach-eval-call is a bit more tricky. Long story short: it loops over all variations (foreach). For each variation v, it expands (call) GEN_rules by replacing $(1) by $(v) (the current variation) and $$ by $. Each expansion result is then instantiated (eval) as a normal make rule. Example: for v=norm, the GEN_rules expansion produces:

norm.twolc: $(patsubst %,%.twolc,$(norm-rules))
    cat $^ > $@

which is in turn expanded as (step-by-step):

step1:

norm.twolc: $(patsubst %,%.twolc,A B C)
    cat $^ > $@

step2:

norm.twolc: A.twolc B.twolc C.twolc
    cat $^ > $@

step3:

norm.twolc: A.twolc B.twolc C.twolc
    cat A.twolc B.twolc C.twolc > norm.twolc

which does what you want: if norm.twolc does not exist or if any of A.twolc, B.twolc, C.twolc is more recent than norm.twolc, the recipe is executed.

code management: generate source files with slight variations of various rules

1 Answers