mirror of
https://github.com/adambard/learnxinyminutes-docs.git
synced 2024-11-29 11:22:14 +03:00
efcd37612d
- nix-repl now deprecated, Nix 2 has `nix repl` - nix language now has floats, and type-preserving arithmetic - add note about immutable sets' attributes being un-redefinable - descendant sets can now have attributes added to them - fix editing loose ene in the Impurity section
378 lines
8.9 KiB
Nix
378 lines
8.9 KiB
Nix
---
|
||
language: nix
|
||
filename: learn.nix
|
||
contributors:
|
||
- ["Chris Martin", "http://chris-martin.org/"]
|
||
- ["Rommel Martinez", "https://ebzzry.io"]
|
||
- ["Javier Candeira", "https://candeira.com/"]
|
||
---
|
||
|
||
Nix is a simple functional language developed for the
|
||
[Nix package manager](https://nixos.org/nix/) and
|
||
[NixOS](https://nixos.org/).
|
||
|
||
You can evaluate Nix expressions using
|
||
[nix-instantiate](https://nixos.org/nix/manual/#sec-nix-instantiate)
|
||
or [`nix repl`](https://nixos.org/nix/manual/#ssec-relnotes-2.0).
|
||
|
||
```
|
||
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 and Floats
|
||
#=========================================
|
||
|
||
# There are two numeric types: integers and floats
|
||
|
||
1 0 42 (-3) # Some integers
|
||
|
||
123.43 .27e13 # A couple of floats
|
||
|
||
# Operations will preserve numeric type
|
||
|
||
(4 + 6 + 12 - 2) # Addition
|
||
#=> 20
|
||
(4 - 2.5)
|
||
#=> 1.5
|
||
|
||
(7 / 2) # Division
|
||
#=> 3
|
||
(7 / 2.0)
|
||
#=> 3.5
|
||
|
||
|
||
# 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; }
|
||
|
||
# Sets are immutable, so you can't redefine an attribute:
|
||
{
|
||
a = { b = 1; };
|
||
a.b = 2;
|
||
}
|
||
#=> attribute 'a.b' at (string):3:5 already defined at (string):2:11
|
||
|
||
# However, an attribute's set members can also be defined piecewise
|
||
# way even if the attribute itself has been directly assigned.
|
||
{
|
||
a = { b = 1; };
|
||
a.c = 2;
|
||
}
|
||
#=> { a = { b = 1; c = 2; }; }
|
||
|
||
|
||
# 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, functional purity is emphasized in the Nix language
|
||
# used to describe Nix packages. 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)
|
||
|
||
* [Rommel Martinez - A Gentle Introduction to the Nix Family]
|
||
(https://ebzzry.io/en/nix/#nix)
|