Hello, Thanks a lot, this is really helpful !
On Thu, Jul 21, 2016 at 5:52 PM, Ricardo Wurmus <rek...@elephly.net> wrote: >> - the comma operator >> - the backquote operator >> - the quote operator >> - the arobase operator (is it for list unpacking ?) > > These are all about list quoting. > This is code: > > (+ 1 2 3) > > It evaluates to 6. > > This is quoted code (aka data): > > '(+ 1 2 3) Those I knew/remembered from my CS student days, it's the other 2 I didn't grok... > It doesn’t get evaluated any further. > This is pretty much the same as: > > `(+ 1 2 3) > > But with the backtick (called “quasiquote”) you are permitted to > temporarily switch from “data mode” to “code mode”. That’s what the > comma (“unquote”) does: > > `(+ 1 2 3 4 ,(+ 3 2) 6) > ^ ^ > data mode code mode > > The result is the same as > > '(+ 1 2 3 4 5 6) > > What’s nice about this is that we can use the same syntax for code that > is to be evaluated right now and for code that we want to pass somewhere > else as inert data which will be evaluated at a later point. > > This allows for “staging”. When you look at a typical package > expression you see that the value of the “arguments” field is quoted. > It is not evaluated right away but in a second stage. > > The value of the inputs field is also quoted. You see that we unquote > the values of package variables there. Package expressions in Guix are > just Scheme values. The inputs field does not merely hold a list of > symbols that somehow represent the packages — it actually contains the > very values themselves! > > “,@” is for splicing lists: That one I guessed, but wasn't sure because of the still unknown comma... Python has something similar, but it does not look as useful as this, especially with that quasiquote thing... > (let ((moo '(1 2 3))) > `(foo bar ,@moo meh)) > > This binds the list '(1 2 3) to the name “moo” and then splices it into > another list. This results in > > '(foo bar 1 2 3 meh) > > Without the “@” and just a regular unquote comma it would be a nested > list: > > (let ((moo '(1 2 3))) > `(foo bar ,moo meh)) > > => '(foo bar (1 2 3) meh) > > Quoting and unquoting is a very useful feature. I hope my explanations > above are easy to understand. Perfect, clear, that was just what I needed. I will not immediately be used to those concepts, but I understand what they do, that's sufficient to read code... The big problem with the abbreviated forms is that they're not easily googlable, if I knew their long names, I could've found them in online doc... >> - the percent operator > > That’s no operator. It’s part of the name. We use the percent as a > prefix for variables that are “special”, e.g. global variables or OK, only a naming convention then ? > values that appear without having to be explicitly let-bound. That could use a little pointer, because the idea is strange, you mean implicit variables (bindings), kind of like perl's $_ ? >> - the #: operator > > These are called keyword arguments. They are no operators either. > Using keyword arguments allows you to specify arguments by name, not in > some fixed order. That I also guessed (I'm coming from **kwargs-land...), but they also look more powerful... >> - the different module import things (#:use-module vs (use-modules) vs ...) > > That’s probably explained in the Guile reference manual. Yes, I read a bit about that in there, but the following is what I wanted to know : > We use “#:use-module” only in module definitions. > “(use-modules …)” can be used anywhere else. > Yeah, I agree. There should be a quick crash course. I spent half the > day yesterday to introduce all these things to a colleague who will > create a few Guix packages in the future. It would be cool to have this > as part of the manual. Thanks a lot, this is just what's missing, and looks like Ludo's covered the .texi part ! -- Vincent Legoll
