diff --git a/pills/05-functions-and-imports.xml b/pills/05-functions-and-imports.xml index cc25225..d15ef59 100644 --- a/pills/05-functions-and-imports.xml +++ b/pills/05-functions-and-imports.xml @@ -4,5 +4,261 @@ version="5.0" xml:id="functions-and-imports"> -functions and imports + functions and imports + +
+ + + Welcome to the fifth Nix pill. In the previous fourth pill we touched the Nix language for a moment. We introduced basic types and values of the Nix language, and basic expressions such as if, with and let. I invite you to re-read about these expressions and play with them in the repl. + + + + Functions help to build reusable components in a big repository like nixpkgs. The Nix manual has a great explanation of functions. Let's go: pill on one hand, Nix manual on the other hand. + + + + I remind you how to enter the Nix environment: source ~/.nix-profile/etc/profile.d/nix.sh + + +
+ +
+ Nameless and single parameter + + + Functions are anonymous (lambdas), and only have a single parameter. The syntax is extremely simple. Type the parameter name, then ":", then the body of the function. + + + + + + So here we defined a function that takes a parameter x, and returns x*2. The problem is that we cannot use it in any way, because it's unnamed... joke! + + + + We can store functions in variables. + + + + + + As usual, please ignore the special syntax for assignments inside nix-repl. So, we defined a function x: x*2 that takes one parameter x, and returns x*2. This function is then assigned to the variable double. Finally we did our first function call: double 3. + + + + Big note: it's not like many other programming languages where you write double(3). It really is double 3. + + + + In summary: to call a function, name the variable, then space, then the argument. Nothing else to say, it's as easy as that. + +
+ +
+ More than one parameter + + + How do we create a function that accepts more than one parameter? For people not used to functional programming, this may take a while to grasp. Let's do it step by step. + + + + + + We defined a function that takes the parameter a, the body returns another function. This other function takes a parameter b and returns a*b. Therefore, calling mul 3 returns this kind of function: b: 3*b. In turn, we call the returned function with 4, and get the expected result. + + + + You don't have to use parenthesis at all, Nix has sane priorities when parsing the code: + + + + + + Much more readable, you don't even notice that functions only receive one argument. Since the argument is separated by a space, to pass more complex expressions you need parenthesis. In other common languages you would write mul(6+7, 8+9). + + + + Given that functions have only one parameter, it is straightforward to use partial application: + + + + + + We stored the function returned by mul 3 into a variable foo, then reused it. + +
+ +
+ Arguments set + + + Now this is a very cool feature of Nix. It is possible to pattern match over a set in the parameter. We write an alternative version of mul = a: b: a*b first by using a set as argument, then using pattern matching. + + + + + + In the first case we defined a function that accepts a single parameter. We then access attributes a and b from the given set. Note how the parenthesis-less syntax for function calls is very elegant in this case, instead of doing mul({ a=3; b=4; }) in other languages. + + + + In the second case we defined an arguments set. It's like defining a set, except without values. We require that the passed set contains the keys a and b. Then we can use those a and b in the function body directly. + + + + + + Only a set with exactly the attributes required by the function is accepted, nothing more, nothing less. + +
+ +
+ Default and variadic attributes + + + It is possible to specify default values of attributes in the arguments set: + + + + + + Also you can allow passing more attributes (variadic) than the expected ones: + + + + + + However, in the function body you cannot access the "c" attribute. The solution is to give a name to the given set with the @-pattern: + + + + + + That's it, you give a name to the whole parameter with name@ before the set pattern. + + + + Advantages of using argument sets: + + + + + + Named unordered arguments: you don't have to remember the order of the arguments. + + + + + You can pass sets, that adds a whole new layer of flexibility and convenience. + + + + + + Disadvantages: + + + + + + Partial application does not work with argument sets. You have to specify the whole attribute set, not part of it. + + + + + + You may find similarities with Python **kwargs. + +
+ +
+ Imports + + + The import function is built-in and provides a way to parse a .nix file. The natural approach is to define each component in a .nix file, then compose by importing these files. + + + + Let's start with the bare metal. + + + + a.nix: + + + + + + b.nix: + + + + + + mul.nix: + + + + + + + + Yes it's really that straight. You import a file, and it gets parsed as expression. Note that the scope of the imported file does not inherit the scope of the importer. + + + + test.nix: + + + + + + + + So how do we pass information to the module? Use functions, like we did with mul.nix. A more complex example: + + + + test.nix: + + + + + + + Explaining: + + + + + + In test.nix we return a function. It accepts a set, with default attributes b, trueMsg and falseMsg. + + + + + builtins.trace is a built-in function that takes two arguments. The first is the message to display, the second is the value to return. It's usually used for debugging purposes. + + + + + Then we import test.nix, and call the function with that set. + + + + + + So when is the message shown? Only when it's in need to be evaluated. + +
+ +
+ Next pill + + + ...we will finally write our first derivation. + +
+ diff --git a/pills/05/a-nix.txt b/pills/05/a-nix.txt new file mode 100644 index 0000000..e440e5c --- /dev/null +++ b/pills/05/a-nix.txt @@ -0,0 +1 @@ +3 \ No newline at end of file diff --git a/pills/05/anon-function.txt b/pills/05/anon-function.txt new file mode 100644 index 0000000..21cadb5 --- /dev/null +++ b/pills/05/anon-function.txt @@ -0,0 +1,2 @@ +nix-repl> x: x*2 +«lambda» \ No newline at end of file diff --git a/pills/05/argument-set-error.txt b/pills/05/argument-set-error.txt new file mode 100644 index 0000000..a741df9 --- /dev/null +++ b/pills/05/argument-set-error.txt @@ -0,0 +1,4 @@ +nix-repl> mul { a = 3; b = 4; c = 6; } +error: anonymous function at (string):1:2 called with unexpected argument `c', at (string):1:1 +nix-repl> mul { a = 3; } +error: anonymous function at (string):1:2 called without required argument `b', at (string):1:1 \ No newline at end of file diff --git a/pills/05/b-nix.txt b/pills/05/b-nix.txt new file mode 100644 index 0000000..bf0d87a --- /dev/null +++ b/pills/05/b-nix.txt @@ -0,0 +1 @@ +4 \ No newline at end of file diff --git a/pills/05/default-values.txt b/pills/05/default-values.txt new file mode 100644 index 0000000..6613ddd --- /dev/null +++ b/pills/05/default-values.txt @@ -0,0 +1,5 @@ +nix-repl> mul = { a, b ? 2 }: a*b +nix-repl> mul { a = 3; } +6 +nix-repl> mul { a = 3; b = 4; } +12 \ No newline at end of file diff --git a/pills/05/import.txt b/pills/05/import.txt new file mode 100644 index 0000000..1defa99 --- /dev/null +++ b/pills/05/import.txt @@ -0,0 +1,5 @@ +nix-repl> a = import ./a.nix +nix-repl> b = import ./b.nix +nix-repl> mul = import ./mul.nix +nix-repl> mul a b +12 \ No newline at end of file diff --git a/pills/05/mul-nix.txt b/pills/05/mul-nix.txt new file mode 100644 index 0000000..5c42a1b --- /dev/null +++ b/pills/05/mul-nix.txt @@ -0,0 +1 @@ +a: b: a*b \ No newline at end of file diff --git a/pills/05/multi-argument-function.txt b/pills/05/multi-argument-function.txt new file mode 100644 index 0000000..8bd584d --- /dev/null +++ b/pills/05/multi-argument-function.txt @@ -0,0 +1,7 @@ +nix-repl> mul = a: (b: a*b) +nix-repl> mul +«lambda» +nix-repl> mul 3 +«lambda» +nix-repl> (mul 3) 4 +12 \ No newline at end of file diff --git a/pills/05/named-function.txt b/pills/05/named-function.txt new file mode 100644 index 0000000..3d61774 --- /dev/null +++ b/pills/05/named-function.txt @@ -0,0 +1,5 @@ +nix-repl> double = x: x*2 +nix-repl> double +«lambda» +nix-repl> double 3 +6 \ No newline at end of file diff --git a/pills/05/named-set-argument.txt b/pills/05/named-set-argument.txt new file mode 100644 index 0000000..1ed840b --- /dev/null +++ b/pills/05/named-set-argument.txt @@ -0,0 +1,3 @@ +nix-repl> mul = s@{ a, b, ... }: a*b*s.c +nix-repl> mul { a = 3; b = 4; c = 2; } +24 \ No newline at end of file diff --git a/pills/05/no-parenthesis.txt b/pills/05/no-parenthesis.txt new file mode 100644 index 0000000..36d12ff --- /dev/null +++ b/pills/05/no-parenthesis.txt @@ -0,0 +1,9 @@ +nix-repl> mul = a: b: a*b +nix-repl> mul +«lambda» +nix-repl> mul 3 +«lambda» +nix-repl> mul 3 4 +12 +nix-repl> mul (6+7) (8+9) +221 \ No newline at end of file diff --git a/pills/05/partial-application.txt b/pills/05/partial-application.txt new file mode 100644 index 0000000..eb925b0 --- /dev/null +++ b/pills/05/partial-application.txt @@ -0,0 +1,5 @@ +nix-repl> foo = mul 3 +nix-repl> foo 4 +12 +nix-repl> foo 5 +15 \ No newline at end of file diff --git a/pills/05/set-argument.txt b/pills/05/set-argument.txt new file mode 100644 index 0000000..792b6f5 --- /dev/null +++ b/pills/05/set-argument.txt @@ -0,0 +1,6 @@ +nix-repl> mul = s: s.a*s.b +nix-repl> mul { a = 3; b = 4; } +12 +nix-repl> mul = { a, b }: a*b +nix-repl> mul { a = 3; b = 4; } +12 \ No newline at end of file diff --git a/pills/05/test-import.txt b/pills/05/test-import.txt new file mode 100644 index 0000000..02b4bd7 --- /dev/null +++ b/pills/05/test-import.txt @@ -0,0 +1,2 @@ +nix-repl> let x = 5; in import ./test.nix +error: undefined variable `x' at /home/lethal/test.nix:1:1 \ No newline at end of file diff --git a/pills/05/test-nix-2.txt b/pills/05/test-nix-2.txt new file mode 100644 index 0000000..cd88a08 --- /dev/null +++ b/pills/05/test-nix-2.txt @@ -0,0 +1,7 @@ +{ a, b ? 3, trueMsg ? "yes", falseMsg ? "no" }: +if a > b + then builtins.trace trueMsg true + else builtins.trace falseMsg false +nix-repl> import ./test.nix { a = 5; trueMsg = "ok"; } +trace: ok +true \ No newline at end of file diff --git a/pills/05/test-nix.txt b/pills/05/test-nix.txt new file mode 100644 index 0000000..c1b0730 --- /dev/null +++ b/pills/05/test-nix.txt @@ -0,0 +1 @@ +x \ No newline at end of file diff --git a/pills/05/variadic-arguments.txt b/pills/05/variadic-arguments.txt new file mode 100644 index 0000000..05fdfec --- /dev/null +++ b/pills/05/variadic-arguments.txt @@ -0,0 +1,2 @@ +nix-repl> mul = { a, b, ... }: a*b +nix-repl> mul { a = 3; b = 4; c = 2; } \ No newline at end of file