Nix gives you two installation options: single user or multi-user. Thanks to laziness, you can get away with these so-called ‘circular imports’ as long as your definitions are not truly circular. The manual provides a referenceof the Nix language. My surname is the same as my father’s. But we can instead implement streams using sets, which are lazy.
Most other languages would have done it in the opposite order. The expression language introduced so far is extremely simple: Notice strings are enclosed in double-quote characters! Python 2.7 vs Python 3.5).
As soon as the outermost thing is no longer an operator but a data constructor (like string, integer, or set), it stops.
But in addition to these fatal errors, Nix also has exceptions, which can be ‘caught’ by a caller. ), $ nix-instantiate --eval --expr '"Hello world"', $ nix-instantiate --eval --expr "'Hello world'", > "Write \\\" to write a literal double-quote", StringChar ::= '\\' | '\"' | Space | Alphanumeric, $ nix-instantiate --eval --expr '42' # integers, $ nix-instantiate --eval ––expr '"Hello " + "world"', $ nix-instantiate --eval ––expr '(400 + 2) * (-5) + (5 * 30)', > (x: y: x*x + y*y) # int -> int -> int, ((x: (y: (x*x + y*y))) 3) 7 # initial expression, > (square: (x: y: square x + square y) 3 7) (x: x*x), > let square=(x: x*x); in (x: y: square x + square y) 3 7, (x: if x < 0 then (-x) else x) (-3) # initial expression, Operator expression Equivalent to, $ nix-instantiate --eval --expr --strict '{ age = 2014-1987; }', { age = 2014–1987; }.age # 1. initial expression, { age: 2014–1987 }.age # 1. initial expression, > builtins.attrNames { age = 26; name = "james"; }, # type stream a = U nil { head=a; tail=stream a; }, > ({x ? Another way is to put all the parameters in a set and pass that as the single argument. We can write a literal list as a whitespace-separated list of other expressions, all in square brackets: Note the absence of commas between the list elements. Though I plan to read all of it, I wanted a quick overview of what I am getting myself into. Nix gives us a way to express this even more compactly using recursive sets. Note also that we can freely mix the types of the elements; integers and strings in the same list poses no problem. Just as with sets, the list elements are left unevaluated unless they are forced. Nix has no type-checking phase (though there are plans for static typing) or any user-facing notion of compilation; it jumps straight to evaluation. It keeps another function, cons, as a private implementation detail. Let’s do some more evaluation. This work by Salar Rahmanian is licensed under a Creative Commons Attribution 4.0 International License. Software Engineer based in San Francisco Bay Area with interests in Scala, Java, Haskell, Moving the packages I have with homebrew to Nix, Salar Rahmanian’s darwin-configuration.nix, Lessons Learned From Being a ZIO Contributor, Life changes and announcing SFBayAreaTech, Creative Commons Attribution 4.0 International License. If for some reason you wanted the arguments to sumOfSquares to default to zero if not provided, we could write: Another way to emulate this behavior is with Nix’s set union operation, written set1 // set2. But how can we access this new module? Paths in the language are always absolute paths. The syntax is slightly unusual, though. (It’s not great style, though.). Sharp-eyed readers will notice I lied earlier when I said that the only thing that happens is ‘evaluation to a value’. So after a successful installation of Nix Package manager I set off installing Nix Darwin: Once Nix and Nix-Darwin are installed you will have new configuration file here: ~/.nixpkgs/darwin-configuration.nix
Using single-quotes will give you an error: So far, the string expressions we’ve seen evaluate to a list of alphanumeric characters and spaces.
To achieve that in timely fashion I read this fantastic overview Adelblog - My journey into Nix by Adelbert Chang. Wait for the sequel of this post, where we learn the power of Nix to comfortably handle multiple environments (e.g. Forcing a subexpression means evaluating it until its top node is a data-type, like an integer, string, or function. Here I show the Nix expression language by example. This project illustrates using the Nix package manager for programming heterogeneous, multi-module projects. Last I run brew cleanup, You can take a look at my configuration here: Salar Rahmanian’s darwin-configuration.nix. The argument does not require parentheses around it, though it doesn’t hurt: All the functions we have seen take a single argument. By this point, we can use Nix as a simple desk calculator: Hmm. Let’s write a function to square an integer: This is the same as (λx. This yields a new, simpler tree: (Alright, I lied again. Following ML, I would write the static type of builtins.div like this: You should mentally parse the -> as right associative, i.e. Second, because it shows Nix’s evaluation order: evaluation in Nix always looks at the top of the tree, where other languages look at the leaves first.
Update: the follow-up blog post is now available, please read Isolated Development Environment with Nix. Nix is a powerful package manager for Linux and other Unix systems that makes package management reliable and reproducible. Strictly speaking, these aren’t trees, since the value ‘3’ has two parents. To encode a literal backslash we use two backslashes: The backslash is used to encode many characters: The expression we gave Nix was really just a value — in particular, a string. There are no facilities for ‘stepping through’ the program execution, and there is no IDE to help with it. So expressions like ‘2014 – 1988’ are left unevaluated because they were not required in order to determine the outermost data-type. We can express the same reduction textually like this: So, why have I gone into such detail here? Yet, those two well-known tools are not the exclusive players. Nix is able to give us sensible runtime type errors like this because values are tagged with their type, as in many other scripting languages. Let’s represent a stream as a two-element set: one element called head, which is the first element of the stream, and another element called tail, which is the rest of the stream encoded as another two-element set.
Again, there are no statements in Nix. You can also generate errors explicitly with the abort builtin: Errors are fatal: if anything evaluates to an error, the entire program stops. It provides atomic upgrades and rollbacks, side-by-side installation of multiple versions of a package, multi-user package management and easy setup of build environments. I was thinking to myself I wish I could do the same above just package management with MacOS. Here are some examples, which are also our first examples of real evaluation: Standard stuff: + is a binary operator which concatenates strings or adds integers.
Nix also creates a profile for every user, which you once you search for an executable (the importance of Nix profile itself will be more obvious once you start to be more familiar with Nix).
For this, Nix gives us ‘sets’. Given these facts, we can define both of us as Nix sets: What devilry is this? For some reason, most programming languages don’t include it, but Nix does. Notice, as always, that the import is an expression, meaning it can go anywhere any other expression can go. Nix Package manager is part of NixOS which is its own Linux based OS. Learn and move my development workflow to using Nix shell. Before performing the + operation, Nix checks the tags of the values. But a nicer way is to use an import to allow us to keep the streams module in a separate file. The james and dad value get repeatedly substituted until there is just addition and attribute lookup to do. We can catch the exception using another builtin called tryEval: The tryEval builtin takes an expression, forces it, and if it throws an exception, returns the set shown above. Ain’t no loops around here. Let’s define a function which takes two integers and returns the sum of their squares. Actually, functions in Nix always take exactly one argument, and multi-argument behavior is achieved via currying. Enter the if-then-else operator: Notice that if-then-else is an expression, not a statement: the whole expression evaluates to either the if-branch or the else-branch depending on whether the condition is true or false. Nix is a powerful package manager for Linux and other Unix systems that makes package management reliable and reproducible. This is not a difficult endeavor: Later on, if you decide that you don’t like the latest version and you prefer to stick with the default one, the rollback leaves no meaningful left-over and it returns the state of the system exactly before you installed Git 2.8: These package management tasks are not unique to Nix. The following is just syntactic sugar for the previous: Now, instead of binding the argument to a single variable, we bind the keys of the argument set as multiple variables in the function body. All the data-types we have seen so far have been primitive data-types. You saw previously how to define multi-parameter functions using currying. The ellipsis at the end of the pattern tells Nix to allow other unused keys in the argument. Earlier I said that functions of multiple parameters can be defined via currying. Whaa? In the second example, because 1 < 2 evaluates to true, the entire expression evaluates to the second value, “icecream”. Nix is a package manager. Well, Nix’s collection of packages (especially on OS X, around seven thousands) is not as impressive as Homebrew and MacPorts. The language can actually be used independently, without any package management at all. Try installing a trivial package like this: Note the installation path, a peculiar subdirectory under /nix/store. Let’s define an function to give us the absolute value of an integer. The segmentation fault was a stack overflow caused by builtins.elemAt trying to fully evaluate the infinite list before doing anything else. Power users on OS X are familiar with Homebrew or MacPorts for installing and managing software packages conveniently. Nix also has familiar operators to manipulate those values. It’s widely regarded as a design mistake, and it was removed from the language in ES5 strict mode. Also be sure not to confuse sets with object-oriented ‘objects’; there are no ‘methods’, no hidden state, and no ‘subtype’ relationships; there is nothing more to a set than its keys and values. Yeah … nope.
Nix is a package manager. They are arrays, with a strict finite length. Each step starts by looking at the top node of the tree. The left child was forced to obtain a function which could be applied to the right child, ‘7’.
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