Question

I am thinking about learning Clojure, but coming from the c-syntax based (java, php, c#) world of imperative languages that’s going to be a challenge, so one naturally asks oneself, is it really worth it? And while such question statement can be very subjective and hard to manage, there is one specific trait of Clojure (and more generally, the lisps) that I keep reading about, that is supposed to make it the most flexipowerful language ever: the macros.

Do you have any good examples of macro usage in Clojure, for purposes which in other mainstream languages (consider any of C++, PHP, Perl, Python, Groovy/Java, C#, JavaScript) would require much less elegant solutions/a lot of unnecessary abstraction/hacks/etc.

Answer 1

there are a lot of macros in the base of clojure that you don’t think about… Which is a sign of a good macro, they let you extend the language in ways that make life easier. With out macros life would be much less exciting. for instance if we didn’t have

(with-out-str (somebody else's code that prints to screen))

then you would need to modify their code in ways that you may not have access to.

another great example is

(with-open-file [fh (open-a-file-code ...)]
   (do (stuff assured that the file won't leak)))

the whole with-something-do pattern of macros have really added to the clojure eco system.

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the other side of the proverbial macro coin is that I spend essentially all of my (current) professional Clojure time using a very macro heavy library and thus i spend a lot of time working around the fact that macros don’t compose well and are not first class. The authors of this library are going to great lengths in the next version to make all the functionality available with out going through the macros to allow people like me to use them in higher order functions like map and reduce.

macros improve the world when they make life easier. They can have the opposite effect when they are the only interface to a library. please don’t use macros as interfaces

It is hard in general to get the shape of your data truly correct. If as a library author, you have the data structured well for how you envision your library being used it may very well be that there is a way to re-structure things to allow users to employ your library in new and unimaginable ways. In this case the structure of the fantastic library in question was really quite good, it allowed for things the authors had not intended. Unfortunatly a great library was restricted because it’s interface was a set of macros not a set of functions. the library was better than it’s macros, so they held it back. This is not to say that macros are in any way to blame, just that programming is hard and they are another tool that can have many effects and all the pieces must be used together to work well.