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diff --git a/doc/manual/src/expressions/expression-syntax.md b/doc/manual/src/expressions/expression-syntax.md new file mode 100644 index 000000000..e3432b577 --- /dev/null +++ b/doc/manual/src/expressions/expression-syntax.md @@ -0,0 +1,93 @@ +# Expression Syntax + +Here is a Nix expression for GNU Hello: + + { stdenv, fetchurl, perl }: ① + + stdenv.mkDerivation { ② + name = "hello-2.1.1"; ③ + builder = ./builder.sh; ④ + src = fetchurl { ⑤ + url = "ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz"; + sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465"; + }; + inherit perl; ⑥ + } + +This file is actually already in the Nix Packages collection in +`pkgs/applications/misc/hello/ex-1/default.nix`. It is customary to +place each package in a separate directory and call the single Nix +expression in that directory `default.nix`. The file has the following +elements (referenced from the figure by number): + +1. This states that the expression is a *function* that expects to be + called with three arguments: `stdenv`, `fetchurl`, and `perl`. They + are needed to build Hello, but we don't know how to build them here; + that's why they are function arguments. `stdenv` is a package that + is used by almost all Nix Packages packages; it provides a + “standard” environment consisting of the things you would expect + in a basic Unix environment: a C/C++ compiler (GCC, to be precise), + the Bash shell, fundamental Unix tools such as `cp`, `grep`, `tar`, + etc. `fetchurl` is a function that downloads files. `perl` is the + Perl interpreter. + + Nix functions generally have the form `{ x, y, ..., + z }: e` where `x`, `y`, etc. are the names of the expected + arguments, and where *e* is the body of the function. So here, the + entire remainder of the file is the body of the function; when given + the required arguments, the body should describe how to build an + instance of the Hello package. + +2. So we have to build a package. Building something from other stuff + is called a *derivation* in Nix (as opposed to sources, which are + built by humans instead of computers). We perform a derivation by + calling `stdenv.mkDerivation`. `mkDerivation` is a function provided + by `stdenv` that builds a package from a set of *attributes*. A set + is just a list of key/value pairs where each key is a string and + each value is an arbitrary Nix expression. They take the general + form `{ + name1 = + expr1; ... + nameN = + exprN; }`. + +3. The attribute `name` specifies the symbolic name and version of the + package. Nix doesn't really care about these things, but they are + used by for instance `nix-env + -q` to show a “human-readable” name for packages. This attribute is + required by `mkDerivation`. + +4. The attribute `builder` specifies the builder. This attribute can + sometimes be omitted, in which case `mkDerivation` will fill in a + default builder (which does a `configure; make; make install`, in + essence). Hello is sufficiently simple that the default builder + would suffice, but in this case, we will show an actual builder + for educational purposes. The value `./builder.sh` refers to the + shell script shown in the [next section](build-script.md), + discussed below. + +5. The builder has to know what the sources of the package are. Here, + the attribute `src` is bound to the result of a call to the + `fetchurl` function. Given a URL and a SHA-256 hash of the expected + contents of the file at that URL, this function builds a derivation + that downloads the file and checks its hash. So the sources are a + dependency that like all other dependencies is built before Hello + itself is built. + + Instead of `src` any other name could have been used, and in fact + there can be any number of sources (bound to different attributes). + However, `src` is customary, and it's also expected by the default + builder (which we don't use in this example). + +6. Since the derivation requires Perl, we have to pass the value of the + `perl` function argument to the builder. All attributes in the set + are actually passed as environment variables to the builder, so + declaring an attribute + + perl = perl; + + will do the trick: it binds an attribute `perl` to the function + argument which also happens to be called `perl`. However, it looks a + bit silly, so there is a shorter syntax. The `inherit` keyword + causes the specified attributes to be bound to whatever variables + with the same name happen to be in scope. |