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The difference between parentheses and curly braces in GNU Make

One of the problems/perks of having written a book about GNU Make is that people ping me with questions. This morning someone said to me: "Especially curly braces vs parentheses is something that always confuses me".

As always the first port of call with GNU Make questions should be the FSF's manual. It says the following: "To substitute a variable’s value, write a dollar sign followed by the name of the variable in parentheses or braces: either $(foo) or ${foo} is a valid reference to the variable foo."

And that seems to work well:

$ cat Makefile
foo := hello.world

$(info $(foo))
$(info ${foo})

$(info $(foo:world=everyone))
$(info ${foo:world=everyone})

$(info $(foo:hello.%=good morning.%))
$(info ${foo:hello.%=good morning.%})

$ make
hello.world
hello.world
hello.everyone
hello.everyone
good morning.world
good morning.world
make: *** No targets.  Stop.

You can see that simple variable references work, as do substitutions (where I changed world to everyone) using substitution references.

But a quick search of the GNU Make manual shows that braces are mentioned once again when talking about function calls: "The delimiters which you use to surround the function call, whether parentheses or braces, can appear in an argument only in matching pairs; the other kind of delimiters may appear singly. If the arguments themselves contain other function calls or variable references, it is wisest to use the same kind of delimiters for all the references; write $(subst a,b,$(x)), not $(subst a,b,${x}). This is because it is clearer, and because only one type of delimiter is matched to find the end of the reference."

The only problem with that description is that the example given using $(subst) works just fine. Here are four possible combinations of parentheses and curly braces:

$ cat Makefile
foo := hello.world

$(info $(subst o,0,$(foo)))
$(info $(subst o,0,${foo}))
$(info ${subst o,0,$(foo)})
$(info ${subst o,0,${foo}})
$ make
hell0.w0rld
hell0.w0rld
hell0.w0rld
hell0.w0rld
make: *** No targets.  Stop.

That worked fine. If you need to work with literal parentheses or braces then the different forms can be helpful. Suppose you want to change every ) to (, or } to {. Here's a Makefile containing two emojis and we use substitution to flip the ) to ( and } to {:

$ cat Makefile
smile := :-)
curly-smile := :-}

$(info $(curly-smile))
$(info $(smile))

$(info $(curly-smile:}={))
$(info $(smile:)=())

$(info ${curly-smile:}={})
$(info ${smile:)=(})
$ make
:-}
:-)
:-{
=()
={}
Makefile:11: *** unterminated variable reference.  Stop.

Wait! That didn't 100% work. The first two $(info) calls did output the value of $(smile) and $(curly-smile) just fine.  Then $(info $(curly-smile:}={)) changed the } to { without a problem because the substitution was made using parentheses (see the start of $(curly-smile:).

But what happened to $(info $(smile:)=()) and why was the output =()? Well, GNU Make interpreted $(smile:) as a variable reference to a variable called smile: and not smile! You can verify that by defining a variable called smile: using this trickery (you have to do that to make a variable called smile: because if you try smile: := foo GNU Make will think you are making a rule to make a file called smile!)

colon := :
smile$(colon) := foo

But in my Makefile above smile: isn't defined so it evaluates to an empty string and we're left with the rest of $(info $(smile:)=()) which is just =() which GNU Make outputs as a string literal. What happened here was GNU Make saw the ) at the end of $(smile:) as closing $(smile:) and not as a literal )

If you want to work with characters that GNU Make treats as special, it's best to define variables to use in their place. Since GNU Make will parse functions calls, variable references etc. before substituting the variables you'll get the output you expect.

$ cat Makefile
smile := :-)
curly-smile := :-}

$(info $(curly-smile))
$(info $(smile))

open-curly := {
close-curly := {
open-paren := (
close-paren := )

$(info $(curly-smile:$(open-curly)=$(close-curl)))
$(info $(smile:$(open-paren)=$(close-paren)))

$(info ${curly-smile:$(open-curly)=$(close-curl)})
$(info ${smile:$(open-paren)=$(close-paren)})

$ make
:-}
:-)
:-}
:-)
:-}
:-)
make: *** No targets.  Stop.

But back to the Makefile above. The last two lines are:

$(info ${curly-smile:}={})
$(info ${smile:)=(})

and the last two lines of output are:

={}
Makefile:11: *** unterminated variable reference.  Stop.

The first line of output is easy to understand. It's the same problem that occurred with $(smile:) but with ${curly-smile:} being interpreted as a reference to a non-existent variable called curly-smile:. But what about the last line? 

Once again GNU Make got confused and parses $(info ${smile:)=(}) as $(info ${smile:) (because the ) terminated the $(info)) followed by =(}).

The moral of this story is... don't mix parentheses and braces in GNU Make, and if you need to work with them as literals then define variables that contain them and use variable references. 

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