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8.5 KiB
| language | filename | contributors | |||
|---|---|---|---|---|---|
| nix | learn.nix |
|
Nix is a simple functional language developed for the Nix package manager and NixOS.
You can evaluate Nix expressions using
nix-instantiate
or nix-repl.
with builtins; [
# Comments
#=========================================
# Inline comments look like this.
/* Multi-line comments
look like this. */
# Booleans
#=========================================
(true && false) # And
#=> false
(true || false) # Or
#=> true
(if 3 < 4 then "a" else "b") # Conditional
#=> "a"
# Integers
#=========================================
# Integers are the only numeric type.
1 0 42 (-3) # Some integers
(4 + 6 + 12 - 2) # Addition
#=> 20
(7 / 2) # Division
#=> 3
# Strings
#=========================================
"Strings literals are in double quotes."
"
String literals can span
multiple lines.
"
''
This is called an "indented string" literal.
It intelligently strips leading whitespace.
''
''
a
b
''
#=> "a\n b"
("ab" + "cd") # String concatenation
#=> "abcd"
# Antiquotation lets you embed values into strings.
("Your home directory is ${getEnv "HOME"}")
#=> "Your home directory is /home/alice"
# Paths
#=========================================
# Nix has a primitive data type for paths.
/tmp/tutorials/learn.nix
# A relative path is resolved to an absolute path at parse
# time, relative to the file in which it occurs.
tutorials/learn.nix
#=> /the-base-path/tutorials/learn.nix
# A path must contain at least one slash, so a relative
# path for a file in the same directory needs a ./ prefix,
./learn.nix
#=> /the-base-path/learn.nix
# The / operator must be surrounded by whitespace if
# you want it to signify division.
7/2 # This is a path literal
(7 / 2) # This is integer division
# Imports
#=========================================
# A nix file contains a single top-level expression with no free
# variables. An import expression evaluates to the value of the
# file that it imports.
(import /tmp/foo.nix)
# Imports can also be specified by strings.
(import "/tmp/foo.nix")
# Import paths must be absolute. Path literals
# are automatically resolved, so this is fine.
(import ./foo.nix)
# But this does not happen with strings.
(import "./foo.nix")
#=> error: string ‘foo.nix’ doesn't represent an absolute path
# Let
#=========================================
# `let` blocks allow us to bind values to variables.
(let x = "a"; in
x + x + x)
#=> "aaa"
# Bindings can refer to each other, and their order does not matter.
(let y = x + "b";
x = "a"; in
y + "c")
#=> "abc"
# Inner bindings shadow outer bindings.
(let a = 1; in
let a = 2; in
a)
#=> 2
# Functions
#=========================================
(n: n + 1) # Function that adds 1
((n: n + 1) 5) # That same function, applied to 5
#=> 6
# There is no syntax for named functions, but they
# can be bound by `let` blocks like any other value.
(let succ = (n: n + 1); in succ 5)
#=> 6
# A function has exactly one argument.
# Multiple arguments can be achieved with currying.
((x: y: x + "-" + y) "a" "b")
#=> "a-b"
# We can also have named function arguments,
# which we'll get to later after we introduce sets.
# Lists
#=========================================
# Lists are denoted by square brackets.
(length [1 2 3 "x"])
#=> 4
([1 2 3] ++ [4 5])
#=> [1 2 3 4 5]
(concatLists [[1 2] [3 4] [5]])
#=> [1 2 3 4 5]
(head [1 2 3])
#=> 1
(tail [1 2 3])
#=> [2 3]
(elemAt ["a" "b" "c" "d"] 2)
#=> "c"
(elem 2 [1 2 3])
#=> true
(elem 5 [1 2 3])
#=> false
(filter (n: n < 3) [1 2 3 4])
#=> [ 1 2 ]
# Sets
#=========================================
# A "set" is an unordered mapping with string keys.
{ foo = [1 2]; bar = "x"; }
# The . operator pulls a value out of a set.
{ a = 1; b = 2; }.a
#=> 1
# The ? operator tests whether a key is present in a set.
({ a = 1; b = 2; } ? a)
#=> true
({ a = 1; b = 2; } ? c)
#=> false
# The // operator merges two sets.
({ a = 1; } // { b = 2; })
#=> { a = 1; b = 2; }
# Values on the right override values on the left.
({ a = 1; b = 2; } // { a = 3; c = 4; })
#=> { a = 3; b = 2; c = 4; }
# The rec keyword denotes a "recursive set",
# in which attributes can refer to each other.
(let a = 1; in { a = 2; b = a; }.b)
#=> 1
(let a = 1; in rec { a = 2; b = a; }.b)
#=> 2
# Nested sets can be defined in a piecewise fashion.
{
a.b = 1;
a.c.d = 2;
a.c.e = 3;
}.a.c
#=> { d = 2; e = 3; }
# An attribute's descendants cannot be assigned in this
# way if the attribute itself has been directly assigned.
{
a = { b = 1; };
a.c = 2;
}
#=> error: attribute ‘a’ already defined
# With
#=========================================
# The body of a `with` block is evaluated with
# a set's mappings bound to variables.
(with { a = 1; b = 2; };
a + b)
# => 3
# Inner bindings shadow outer bindings.
(with { a = 1; b = 2; };
(with { a = 5; };
a + b))
#=> 7
# This first line of tutorial starts with "with builtins;"
# because builtins is a set the contains all of the built-in
# functions (length, head, tail, filter, etc.). This saves
# us from having to write, for example, "builtins.length"
# instead of just "length".
# Set patterns
#=========================================
# Sets are useful when we need to pass multiple values
# to a function.
(args: args.x + "-" + args.y) { x = "a"; y = "b"; }
#=> "a-b"
# This can be written more clearly using set patterns.
({x, y}: x + "-" + y) { x = "a"; y = "b"; }
#=> "a-b"
# By default, the pattern fails on sets containing extra keys.
({x, y}: x + "-" + y) { x = "a"; y = "b"; z = "c"; }
#=> error: anonymous function called with unexpected argument ‘z’
# Adding ", ..." allows ignoring extra keys.
({x, y, ...}: x + "-" + y) { x = "a"; y = "b"; z = "c"; }
#=> "a-b"
# Errors
#=========================================
# `throw` causes evaluation to abort with an error message.
(2 + (throw "foo"))
#=> error: foo
# `tryEval` catches thrown errors.
(tryEval 42)
#=> { success = true; value = 42; }
(tryEval (2 + (throw "foo")))
#=> { success = false; value = false; }
# `abort` is like throw, but it's fatal; it cannot be caught.
(tryEval (abort "foo"))
#=> error: evaluation aborted with the following error message: ‘foo’
# `assert` evaluates to the given value if true;
# otherwise it throws a catchable exception.
(assert 1 < 2; 42)
#=> 42
(assert 1 > 2; 42)
#=> error: assertion failed at (string):1:1
(tryEval (assert 1 > 2; 42))
#=> { success = false; value = false; }
# Impurity
#=========================================
# Because repeatability of builds is critical to the Nix package
# manager, in which, functional purity is emphasized in the Nix
# language. But there are a few impurities.
# You can refer to environment variables.
(getEnv "HOME")
#=> "/home/alice"
# The trace function is used for debugging. It prints the first
# argument to stderr and evaluates to the second argument.
(trace 1 2)
#=> trace: 1
#=> 2
# You can write files into the Nix store. Although impure, this is
# fairly safe because the file name is derived from the hash of
# its contents. You can read files from anywhere. In this example,
# we write a file into the store, and then read it back out.
(let filename = toFile "foo.txt" "hello!"; in
[filename (builtins.readFile filename)])
#=> [ "/nix/store/ayh05aay2anx135prqp0cy34h891247x-foo.txt" "hello!" ]
# We can also download files into the Nix store.
(fetchurl "https://example.com/package-1.2.3.tgz")
#=> "/nix/store/2drvlh8r57f19s9il42zg89rdr33m2rm-package-1.2.3.tgz"
]
Further Reading
-
[Nix Manual - Nix expression language] (https://nixos.org/nix/manual/#ch-expression-language)
-
[James Fisher - Nix by example - Part 1: The Nix expression language] (https://medium.com/@MrJamesFisher/nix-by-example-a0063a1a4c55)
-
[Susan Potter - Nix Cookbook - Nix By Example] (http://funops.co/nix-cookbook/nix-by-example/)