Why doesn't Make allow indented "if" statements?

Question

It's a constant thorn in my side when trying to read a makefile with nested logic, that Make does not allow indented if statements. Why is this, and is there a good way to work around this limitation, and still have readable makefiles?

Update: I now realise that this question is based on a false premise, but I believe that leaving it here may be valuable to anyone who makes the same mistake that I did.

Answer 1

Thanks to the help of others, I now realize that my question is posed on a false premise. Makefiles absolutely do allow for indented if statements, or rather indented conditionals to be more precise. What they don't allow for - at least out of the box - are tabbed conditionals. This is because, by default, Make interprets tabs as especially meaningful characters. Almost any line beginning with a tab character is interpreted to be part of a recipe. Therefore, any line that is not intended to be part of a recipe - such as conditionals - should not begin with a tab.

As far as answering the part of my question that asked why they chose to use the tab character in this way, I haven't found an answer. Perhaps the designers intended for conditionals to be used sparingly.

As for workarounds, here I will attempt to describe a couple.

The first solution is a terrible pain if you don't have an editor that shows whitespace characters, but if you do, the simplest thing to do might be to just add some spaces to indent your non recipe code. This is a rather hackish workaround though and probably ill advised.

Another solution (courtesy of @Stefan Becker) is to set the special variable, .RECIPEPREFIX to a character other than tab. Here's an example of what I tried:

.RECIPEPREFIX := >
# Now, all recipes will begin with the > character rather than a tab.

things = something another_thing something_else nothing
nothing = true

something: another_thing something_else
# See how each line of a recipe now begins with >.
# You can see I also added a tab after the >. 
# These tabs doesn't mean anything to Make; it's just for readability.
>   $(info Making $@.)
>   @touch $@

another_thing:
>   $(info Making $@.)
    # See also how lines like comments can be tabbed, 
    # but no longer add anything meaningful to recipes.
>   @touch $@

something_else:
>   $(info Making $@.)
>   @touch $@
    # And just to prove the situation with conditionals is resolved...
    # See how the @touch command begins with the new RECIPEPREFIX 
    # but the conditionals don't.
    ifeq ($(nothing),true)
>       $(info Also making nothing, because nothing is true.)
>       @touch nothing
    endif

.PHONY: everything_clean
everything_clean:
>   $(info Cleaning up everything.)
>   rm -f $(things)

One thing worth remembering is that recipe lines must begin with the new RECIPEPREFIX. That is to say, that something like this won't work:

something: another_thing something_else
# Remember that the RECIPEPREFIX must come first. 
# Indenting your recipe lines first and then using the RECIPEPRIFX will not work.
    >$(info Making $@.)
    >@touch $@

Answer 2

I don't know why you are under the impression that indented conditionals aren't supported. They do seem to work fine when I use them in the following example:

.PHONY: all
all:

CONFIGS :=

ifeq ($(CONFIG1),1)
  $(info CONFIG1 selected)

  CONFIGS += config1
  all: config1

  config1:
    @echo $@


  ifeq ($(CONFIG2),1)
    $(info CONFIG2 selected)

    CONFIGS += config2
    all: config2

    config2:
    @echo $@

  else
    $(info CONFIG2 not selected)
  endif
else
  $(info CONFIG1 NOT selected)
endif

all:
    @echo "all: $(CONFIGS)"

NOTE: the TABS in my example will probably not survive copy & paste. So you'll have to re-enter them for the recipes.

Test run:

$ make
CONFIG1 NOT selected
all:

$ make CONFIG1=1
CONFIG1 selected
CONFIG2 not selected
config1
all:  config1

$ make CONFIG1=1 CONFIG2=1
CONFIG1 selected
CONFIG2 selected
config1
config2
all:  config1 config2

---

But...

There is one case where indentation can lead to problems. To quote the GNU make manual:

A recipe is an action that make carries out. A recipe may have more than one command, either on the same line or each on its own line. Please note: you need to put a tab character at the beginning of every recipe line! This is an obscurity that catches the unwary.

As GNU make takes all TAB indented lines after a rule to be part of the recipe for the rule the following will fail for make CONFIG1=1:

.PHONY: all
all:

CONFIGS :=

config1:
# TAB in the following line
    @echo $@

# the following lines are indented with TABs
    ifeq ($(CONFIG1),1)
    CONFIGS += config1
    test1:
        @echo $@
    endif

ifeq ($(CONFIG1),1)
all: config1
endif

all:
# TAB in the following line
    @echo "all: $(CONFIGS)"

$ make CONFIG1=1
config1
ifeq (1,1)
/bin/sh: -c: line 0: syntax error near unexpected token `1,1'
/bin/sh: -c: line 0: `ifeq (1,1)'
make: *** [Makefile:9: config1] Error 1

Solution

1. organize the makefile to have conditionals first, then rules, i.e. no TAB indentation after rules anymore except for recipes, or
2. always make sure to use SPACEs for conditional, variable assignment and rule lines.
3. set .RECIPEPREFIX to a non-whitespace character, e.g. > and use that to indicate recipe lines.¹

Unless you have an editor which shows the difference between TABs and SPACEs, alternative 2 will probably drive you insane. I would suggest alternative 1 instead...

The following works for make CONFIG2=1:

.PHONY: all
all:

CONFIGS :=

config2:
# TAB in the following line
    @echo $@

# the following lines are indented with SPACES
    ifeq ($(CONFIG2),1)
    CONFIGS += config2
    test2:
# 2 TABs in the following line
        @echo $@
    endif

ifeq ($(CONFIG2),1)
all: config2
endif

all:
# TAB in the following line
    @echo "all: $(CONFIGS)"

$ make CONFIG2=1
config2
all:  config2

---

¹ you might be tempted to set .RECIPEPREFIX to SPACE like this:

_empty        :=
_space        := $(_empty) $(_empty)
.RECIPEPREFIX := $(_space)

and then switch your editor to use only SPACEs. But that makes things worse, i.e. now make can't distinguish between normal and recipe indentation. If you try this with the above example you will note that it now fails for any invocation that enables one of the indented rules.