Nix Language Quirks: Difference between revisions

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imported>Danbst
added function argument destructuring
imported>Danbst
added imports and namespacing section
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These <code>x</code> and <code>y</code> are in fact <code>let</code>-bindings, but overridable ones.
These <code>x</code> and <code>y</code> are in fact <code>let</code>-bindings, but overridable ones.
== Imports and namespaces ==
There is a keyword <code>import</code>, but it's equivalent in other languages is  <code>eval</code>. It can be used for namespacing too:
<nowiki>let
  pkgs = import <nixpkgs> {};
  lib = import <nixpkgs/lib>;
in
  pkgs.runCommand (lib.strings.removePrefix "....</nowiki>
consider using <code>import</code> here as using <code>qualified import ...</code> in Haskell or <code>import ...</code> in Python.
Another way of importing is <code>with import ...;</code>, which corresponds to Python <code>from ... import *</code>.
But because of not very great IDE support in Nix, <code>with import ...;</code> is discouraged. Rather use <code>inherit</code>, especially if you are targeting source code for Nix newcomers:
<nowiki>
let
  lib = import <nixpkgs/lib>;
  inherit (lib.strings)
    removePrefix removeSuffix
  ;
  inherit (lib.lists)
    isList init drop
  ;
in
  removePrefix ...</nowiki>
<code>inherit</code> has higher priority than <code>with</code>, and conflicts with <code>let</code>
<nowiki>
nix-repl> let pkgs = { x = 1; }; x = 2; x = 3; inherit (pkgs) x; in x
error: attribute ‘x’ at (string):1:31 already defined at (string):1:24</nowiki>
This makes it a sane citizen of Nix lanugage... except it has a twin, called <code>{ inherit ...; }</code>. They DON'T do the same - <code>let inherit ...</code> adds let-bindings, and <code>{ inherit ...; }</code> adds attributes to a record.

Revision as of 11:11, 26 October 2017

with and let

with gets less priority then let. This can lead to confusions, especially if you like to write with pkgs;:

nix-repl> pkgs = { x = 1; }

nix-repl> with pkgs; x
1

nix-repl> with pkgs; let x = 2; in x
2

So we see, that let binding overrides with binding. But what about this?

nix-repl> let x = 2; in with pkgs; x
2

Nah, with and let have different priority when resolving names.

Good discussion on this topic

Old let syntax

This is an old Nix syntax, that probably isn't used much

nix-repl> let { x = 1; y = x + 1; body = y; }
2

It is equivalent to modern syntax expression let x = 1; y = x + 1; in y. Note, that it doesn't require rec keyword.

Note, that it isn't equivalent to with rec { x = 1; y = x + 1; body = y; }; body because of mentioned with and let quirk, but is same as rec { x = 1; y = x + 1; body = y; }.body

Something that looks like both record attribute and let-binding

Destructuring function argument - is a great feature of Nix.

nix-repl> f = { x ? 1, y ? 2 }: x + y

nix-repl> f { }
3

The fact that we can add @args argument assignment is also cool

nix-repl> f = { x ? 1, y ? 2, ... }@args: with args; x + y + z

nix-repl> f { z = 3; }
6

But don't be fooled, args doesn't necessarily contain x and y:

nix-repl> f = { x ? 1, y ? 2, ... }@args: args.x + args.y + args.z

nix-repl> f { z = 3;}
error: attribute ‘x’ missing, at (string):1:30

These x and y are in fact let-bindings, but overridable ones.

Imports and namespaces

There is a keyword import, but it's equivalent in other languages is eval. It can be used for namespacing too:

let
  pkgs = import <nixpkgs> {};
  lib = import <nixpkgs/lib>;
in
  pkgs.runCommand (lib.strings.removePrefix "....

consider using import here as using qualified import ... in Haskell or import ... in Python.

Another way of importing is with import ...;, which corresponds to Python from ... import *.

But because of not very great IDE support in Nix, with import ...; is discouraged. Rather use inherit, especially if you are targeting source code for Nix newcomers:

let
  lib = import <nixpkgs/lib>;
  inherit (lib.strings)
    removePrefix removeSuffix
  ;
  inherit (lib.lists)
    isList init drop
  ;
in
  removePrefix ...

inherit has higher priority than with, and conflicts with let

nix-repl> let pkgs = { x = 1; }; x = 2; x = 3; inherit (pkgs) x; in x
error: attribute ‘x’ at (string):1:31 already defined at (string):1:24

This makes it a sane citizen of Nix lanugage... except it has a twin, called { inherit ...; }. They DON'T do the same - let inherit ... adds let-bindings, and { inherit ...; } adds attributes to a record.