Nix Language Quirks: Difference between revisions

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{{merge|Nix ~~Expression Language|same type of information in the expression~~ language ~~article~~}}

== <code>with</code> and <code>let</code> ==

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2 </syntaxHighlight>

~~Nah~~, <code>with</code> and <code>let</code> have different priority when resolving names.

In this case, <code>with</code> and <code>let</code> have different priority when resolving names.

[https://github.com/NixOS/nix/issues/1361 Good discussion on this topic]

Generally the use of <code>with</code> is discouraged. See the [https://nix.dev/guides/best-practices#with-scopes best practices guide] for how best to use <code>inherit</code> as an alternative.

== Old <code>let</code> syntax ==

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

This is an [https://github.com/NixOS/nix/issues/1361#issuecomment-323050690 old] Nix syntax, that probably isn't used much

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

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

== Default values are not bound in @ syntax ==

== Default values are not bound in <code>@</code> syntax ==

Destructured arguments can have default values, but those default values are part of the full function argument.

In the following example, calling the function that binds a default value <code>"a"</code> to the argument's attribute <code>a</code> with an empty attribute set as argument will produce an empty attribute set <code>args</code> instead of <code>{ a = "a"; }</code>:

In the following example, calling the function that binds a default value <code>"a"</code> to the argument's attribute <code>a</code> with an empty attribute set as an argument will produce an empty attribute set <code>args</code> instead of <code>{ a = "a"; }</code>:

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== 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~~:

Nix includes a keyword <code>import</code>, but it's equivalent in other languages is <code>eval</code>.

It is typically be used for namespacing:

<syntaxHighlight lang=nix>let

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pkgs.runCommand (lib.strings.removePrefix ".... </syntaxHighlight>

consider ~~using~~ <code>import</code> here as using <code>qualified import ...</code> in Haskell or <code>import ...</code> in Python.

consider the use of <code>import</code> here similar to using <code>qualified import ...</code> in Haskell or <code>import ...</code> in Python. Another (discouraged and increasingly uncommon) way of importing is [https://nix.dev/manual/nix/2.24/language/syntax#with-expressions <code>with import ...;</code>], which corresponds to Python <code>from ... import *</code>. This use of <code>with</code> imports everything from the target into scope, which has numerous potential gotchas and problems, and so using [https://nix.dev/guides/best-practices#with-scopes <code>inherit</code>] instead is encouraged and preferred.

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:~~

~~<syntaxHighlight lang=nix>~~

~~let~~

~~lib = import <nixpkgs/lib>;~~

~~inherit (lib~~.~~strings)~~

~~removePrefix removeSuffix~~

;

~~inherit (lib.lists)~~

~~isList init drop~~

;

in

~~removePrefix ... </syntaxHighlight>~~

~~<code>inherit</code> has higher priority than <code>with</code>, and conflicts with <code>let</code>~~

== <code>builtins.replaceStrings</code> key match on "" ==

~~<syntaxHighlight lang=nix>~~

~~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 </syntaxHighlight>~~

~~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.~~

~~== builtins.replaceStrings~~ key match on "" ==

Syntax:

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</syntaxHighlight>

~~Function~~ allows ~~match for~~ "" in string. <code>[match]</code> gets checked sequentially, and when "" is checked - it always matches. And so - when "" is checked in always inserts the replacement, then next char in ~~sting~~ gets ~~passed through~~, and the next char after that from ~~the~~ string gets processed.

The [https://noogle.dev/f/builtins/replaceStrings <code>builtins.replaceStrings</code>] function allows matching <code>""</code> in <code>string</code>. <code>[match]</code> gets checked sequentially, and when <code>""</code> is checked - it ''always'' matches. And so - when <code>""</code> is checked it ''always'' inserts the corresponding replacement from <code>[replace]</code>, then the next char in <code>string</code> gets inserted, and then the next char after that from <code>string</code> gets processed.

nix-repl> builtins.replaceStrings ["" "e"] [" " "i"] "Hello world"

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</syntaxHighlight>

== Indented ~~String trims~~ leading whitespace ==

== Indented strings trim leading whitespace ==

~~Not really surprising, but ...~~

Leading spaces are removed from both single-line and multi-line <strong>indented strings</strong>.

Leading ~~whitespace is~~ removed ~~also in~~ single-line ~~Indented Strings~~

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</syntaxHighlight>

Usually, ~~Indented Strings~~ have multiple lines

Usually, indented strings have multiple lines:

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'' == "s\n"

</syntaxHighlight>

Though note that [https://en.wikipedia.org/wiki/Tab_key#Tab_characters tab characters] are ''not'' stripped:

''

s

'' == " s\n"

</syntaxhighlight>

This is documented in more detail in the [https://nix.dev/manual/nix/2.24/language/syntax#string-literal String section] of the Nix reference manual. Also see [https://github.com/NixOS/nix/issues/7834 NixOS/nix#7834] and [https://github.com/NixOS/nix/pull/9971 NixOS/nix#9971] for more information.

== Integer precision ==

Integer precision is limited to 64 ~~Bit~~ in the original Nix interpreter.

Integer precision is limited to [https://en.m.wikipedia.org/wiki/64-bit_computing 64-bit] in the original Nix interpreter.

So the valid integer range is from -2**63 to 2**63-1 = from -9223372036854775808 to 9223372036854775807

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error: invalid integer '9223372036854775808'

</syntaxHighlight>

== No negative number literals ==

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nix-repl> -9223372036854775807 - 1

-9223372036854775808

</syntaxHighlight>

== Attribute set entries with a name that evaluates to null will not be added to the set ==

From [https://nix.dev/manual/nix/2.24/language/syntax#attrs-literal this] section of the Nix Reference Manual:

In the special case where an attribute name inside of a set declaration evaluates to null (which is normally an error, as null cannot be coerced to a string), that attribute is simply not added to the set:

{ ${if foo then "bar" else null} = true; }

This will evaluate to {} if foo evaluates to false.

</blockquote>

The relevant source can be found [https://github.com/NixOS/nix/blob/26c3fc11eada3fa7df0284190095868a947fefe2/src/libexpr/eval.cc#L1249-L1250 here].

This feature can be used to conditionally include or exclude attribute set entries, for example:

nix-repl> { ${if true then "foo" else null} = "bar"; }

{ foo = "bar"; }

nix-repl> { ${if false then "foo" else null} = "bar"; }

{ }

</syntaxHighlight>

This might be used as an alternative to conditionally merging attribute sets using <code>//</code> like the following:

{ a = "b"; } // (if true then { foo = "bar"; } else { } )

</syntaxHighlight>

== Hexadecimal, octal, and binary ==

As of late 2024, Nix doesn't contain builtin support for parsing many number formats like hexadecimal, octal, and binary. It ''does'', however, support the [https://noogle.dev/f/builtins/fromTOML <code>builtins.fromTOML</code>] function, which [https://github.com/NixOS/nix/issues/7578#issuecomment-1955985859 can be used] to parse these number formats.

nix-repl> (builtins.fromTOML "octal = 0o11").octal

9

nix-repl> (builtins.fromTOML "binary = 0b1001").binary

9

nix-repl> (builtins.fromTOML "hex = 0x09").hex

9

</syntaxHighlight>

== Mimicking case statements with attribute sets ==

Nix doesn't include native support for case statements, however when dealing with string types it's possible to use some string interpolation behavior to achieve something similar to case statement behavior, as described in this [https://discourse.nixos.org/t/case-statement-expr/27741/12 thread].

In the example from the thread, given some string argument <code>x</code>, the following code would place different values into a text file depending on it's value:

environment.etc."just/for/test".text = {

"a" = "hello";

"b" = "hi";

"c" = "ciao";

}."${x}";

</syntaxHighlight>

So if <code>x</code> is set to the string <code>"a"</code> then the <code>just/for/test</code> file contents would be set to the string <code>"hello"</code>. The code above is the same in behavior to the following, more common, if-else-style construct:

environment.etc."just/for/test".text =

if x == "a" then

"hello"

else if x == "b" then

"hi"

else if x == "c" then

"ciao"

else

abort "x is invalid";

</syntaxHighlight>

There is an example in <code>coq</code> package code [https://github.com/NixOS/nixpkgs/blob/5185539c51ba658e70b29e01c0c320a85f4e2098/pkgs/build-support/coq/extra-lib.nix#L98 here] where someone used this behavior to build a reusable function <code>switch</code>.

== <code>builtins.toString</code> handling of <code>true</code> and <code>false</code> is inconsistent ==

nix-repl> builtins.toString true

"1"

nix-repl> builtins.toString false

""

</syntaxHighlight>

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{{note|[https://github.com/NixOS/nix/commit/f90f660b243866b8860eeb24cc4a345d32cc7ce7 Nix 1.12 should fix this]}}

=== Q: Can Nix code be interpolated? ===

~~Yes~~, ~~but not everywhere~~

No, only attribute names can.

nix-repl> let ${"x"} = 2; in x

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=== Q: Can it be <code>eval</code>-ed from string? ===

A: ~~Sure~~, but only via "import from derivation":

A: Yes, but it is not recommended as "eval" is generally regarded as an easy to abuse language feature. It is possible but only via the store (not as bad as "import from derivation", but still not suitable for hot code paths):

nix-repl> let code = "(x: x) ''id function was called''"; in import (builtins.toFile "eval" code)

"id function was called"</syntaxHighlight>

= Resources =

* [https://md.darmstadt.ccc.de/xtNP7JuIQ5iNW1FjuhUccw# A separately maintained list of Nix language quirks]

[[Category:Nix Language]]

@@ Line 1: / Line 1: @@
-{{merge|Nix Expression Language|same type of information in the expression language article}}
+{{merge|Nix (language)}}
 == <code>with</code> and <code>let</code> ==
@@ Line 20: / Line 20: @@
 </syntaxHighlight>
-Nah, <code>with</code> and <code>let</code> have different priority when resolving names.
+In this case, <code>with</code> and <code>let</code> have different priority when resolving names.
 [https://github.com/NixOS/nix/issues/1361 Good discussion on this topic]
+Generally the use of <code>with</code> is discouraged. See the [https://nix.dev/guides/best-practices#with-scopes best practices guide] for how best to use <code>inherit</code> as an alternative.
 == Old <code>let</code> syntax ==
-This is an old Nix syntax, that probably isn't used much
+This is an [https://github.com/NixOS/nix/issues/1361#issuecomment-323050690 old] Nix syntax, that probably isn't used much
   <syntaxHighlight lang=nix>
 nix-repl> let { x = 1; y = x + 1; body = y; }
@@ Line 34: / Line 36: @@
 Note, that it isn't equivalent to <code>with rec { x = 1; y = x + 1; body = y; }; body</code> because of mentioned <code>with</code> and <code>let</code> quirk, but is same as <code>rec { x = 1; y = x + 1; body = y; }.body</code>
-== Default values are not bound in @ syntax ==
+== Default values are not bound in <code>@</code> syntax ==
 Destructured arguments can have default values, but those default values are part of the full function argument.
-In the following example, calling the function that binds a default value <code>"a"</code> to the argument's attribute <code>a</code> with an empty attribute set as argument will produce an empty attribute set <code>args</code> instead of <code>{ a = "a"; }</code>:
+In the following example, calling the function that binds a default value <code>"a"</code> to the argument's attribute <code>a</code> with an empty attribute set as an argument will produce an empty attribute set <code>args</code> instead of <code>{ a = "a"; }</code>:
 <syntaxHighlight lang=nix>
@@ Line 73: / Line 75: @@
 == 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:
+Nix includes a keyword <code>import</code>, but it's equivalent in other languages is <code>eval</code>.
+It is typically be used for namespacing:
   <syntaxHighlight lang=nix>let
@@ Line 81: / Line 85: @@
    pkgs.runCommand (lib.strings.removePrefix ".... </syntaxHighlight>
-consider using <code>import</code> here as using <code>qualified import ...</code> in Haskell or <code>import ...</code> in Python.
+consider the use of <code>import</code> here similar to using <code>qualified import ...</code> in Haskell or <code>import ...</code> in Python. Another (discouraged and increasingly uncommon) way of importing is [https://nix.dev/manual/nix/2.24/language/syntax#with-expressions <code>with import ...;</code>], which corresponds to Python <code>from ... import *</code>. This use of <code>with</code> imports everything from the target into scope, which has numerous potential gotchas and problems, and so using [https://nix.dev/guides/best-practices#with-scopes <code>inherit</code>] instead is encouraged and preferred.
-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:
-  <syntaxHighlight lang=nix>
-let
-  lib = import <nixpkgs/lib>;
-  inherit (lib.strings)
-    removePrefix removeSuffix
-  ;
-  inherit (lib.lists)
-    isList init drop
-  ;
-in
-  removePrefix ... </syntaxHighlight>
-<code>inherit</code> has higher priority than <code>with</code>, and conflicts with <code>let</code>
+== <code>builtins.replaceStrings</code> key match on "" ==
-  <syntaxHighlight lang=nix>
-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 </syntaxHighlight>
-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.
-== builtins.replaceStrings key match on "" ==
 Syntax:
@@ Line 113: / Line 94: @@
 </syntaxHighlight>
-Function allows match for "" in string. <code>[match]</code> gets checked sequentially, and when "" is checked - it always matches. And so - when "" is checked in always inserts the replacement, then next char in sting gets passed through, and the next char after that from the string gets processed.
+The [https://noogle.dev/f/builtins/replaceStrings <code>builtins.replaceStrings</code>] function allows matching <code>""</code> in <code>string</code>. <code>[match]</code> gets checked sequentially, and when <code>""</code> is checked - it ''always'' matches. And so - when <code>""</code> is checked it ''always'' inserts the corresponding replacement from <code>[replace]</code>, then the next char in <code>string</code> gets inserted, and then the next char after that from <code>string</code> gets processed.
 <syntaxHighlight lang=nix>
 nix-repl> builtins.replaceStrings ["" "e"] [" " "i"] "Hello world"
@@ Line 121: / Line 102: @@
 </syntaxHighlight>
-== Indented String trims leading whitespace ==
+== Indented strings trim leading whitespace ==
-Not really surprising, but ...
+Leading spaces are removed from both single-line and multi-line <strong>indented strings</strong>.
-Leading whitespace is removed also in single-line Indented Strings
 <syntaxHighlight lang=nix>
@@ Line 131: / Line 110: @@
 </syntaxHighlight>
-Usually, Indented Strings have multiple lines
+Usually, indented strings have multiple lines:
 <syntaxHighlight lang=nix>
@@ Line 138: / Line 117: @@
 '' == "s\n"
 </syntaxHighlight>
+Though note that [https://en.wikipedia.org/wiki/Tab_key#Tab_characters tab characters] are ''not'' stripped:
+<syntaxhighlight lang=nix>
+''
+	s
+'' ==  "	s\n"
+</syntaxhighlight>
+This is documented in more detail in the [https://nix.dev/manual/nix/2.24/language/syntax#string-literal String section] of the Nix reference manual. Also see [https://github.com/NixOS/nix/issues/7834 NixOS/nix#7834] and [https://github.com/NixOS/nix/pull/9971 NixOS/nix#9971] for more information.
 == Integer precision ==
-Integer precision is limited to 64 Bit in the original Nix interpreter.
+Integer precision is limited to [https://en.m.wikipedia.org/wiki/64-bit_computing 64-bit] in the original Nix interpreter.
 So the valid integer range is from -2**63 to 2**63-1 = from -9223372036854775808 to 9223372036854775807
@@ Line 158: / Line 147: @@
 error: invalid integer '9223372036854775808'
 </syntaxHighlight>
+<!-- TODO Float precision -->
-<!-- TODO Float Precision -->
 == No negative number literals ==
@@ Line 182: / Line 170: @@
 nix-repl> -9223372036854775807 - 1
 -9223372036854775808
+</syntaxHighlight>
+== Attribute set entries with a name that evaluates to null will not be added to the set  ==
+From [https://nix.dev/manual/nix/2.24/language/syntax#attrs-literal this] section of the Nix Reference Manual:
+<blockquote>
+In the special case where an attribute name inside of a set declaration evaluates to null (which is normally an error, as null cannot be coerced to a string), that attribute is simply not added to the set:
+{ ${if foo then "bar" else null} = true; }
+This will evaluate to {} if foo evaluates to false.
+</blockquote>
+The relevant source can be found [https://github.com/NixOS/nix/blob/26c3fc11eada3fa7df0284190095868a947fefe2/src/libexpr/eval.cc#L1249-L1250 here].
+This feature can be used to conditionally include or exclude attribute set entries, for example:
+<syntaxHighlight lang=nix>
+nix-repl> { ${if true then "foo" else null} = "bar"; }
+{ foo = "bar"; }
+nix-repl> { ${if false then "foo" else null} = "bar"; }
+{ }
+</syntaxHighlight>
+This might be used as an alternative to conditionally merging attribute sets using <code>//</code> like the following:
+<syntaxHighlight lang=nix>
+{ a = "b"; } // (if true then { foo = "bar"; } else { } )
+</syntaxHighlight>
+== Hexadecimal, octal, and binary ==
+As of late 2024, Nix doesn't contain builtin support for parsing many number formats like hexadecimal, octal, and binary. It ''does'', however, support the [https://noogle.dev/f/builtins/fromTOML <code>builtins.fromTOML</code>] function, which [https://github.com/NixOS/nix/issues/7578#issuecomment-1955985859 can be used] to parse these number formats.
+<syntaxHighlight lang=nix>
+nix-repl> (builtins.fromTOML "octal = 0o11").octal
+nix-repl> (builtins.fromTOML "binary = 0b1001").binary
+nix-repl> (builtins.fromTOML "hex = 0x09").hex
+</syntaxHighlight>
+== Mimicking case statements with attribute sets ==
+Nix doesn't include native support for case statements, however when dealing with string types it's possible to use some string interpolation behavior to achieve something similar to case statement behavior, as described in this [https://discourse.nixos.org/t/case-statement-expr/27741/12 thread].
+In the example from the thread, given some string argument <code>x</code>, the following code would place different values into a text file depending on it's value:
+<syntaxHighlight lang=nix>
+environment.etc."just/for/test".text = {
+  "a" = "hello";
+  "b" = "hi";
+  "c" = "ciao";
+}."${x}";
+</syntaxHighlight>
+So if <code>x</code> is set to the string <code>"a"</code> then the <code>just/for/test</code> file contents would be set to the string <code>"hello"</code>. The code above is the same in behavior to the following, more common, if-else-style construct:
+<syntaxHighlight lang=nix>
+environment.etc."just/for/test".text =
+  if x == "a" then
+    "hello"
+  else if x == "b" then
+    "hi"
+  else if x == "c" then
+    "ciao"
+  else
+    abort "x is invalid";
+</syntaxHighlight>
+There is an example in <code>coq</code> package code [https://github.com/NixOS/nixpkgs/blob/5185539c51ba658e70b29e01c0c320a85f4e2098/pkgs/build-support/coq/extra-lib.nix#L98 here] where someone used this behavior to build a reusable function <code>switch</code>.
+== <code>builtins.toString</code> handling of <code>true</code> and <code>false</code> is inconsistent ==
+<syntaxHighlight lang=nix>
+nix-repl> builtins.toString true
+"1"
+nix-repl> builtins.toString false
+""
 </syntaxHighlight>
@@ Line 200: / Line 273: @@
 {{note|[https://github.com/NixOS/nix/commit/f90f660b243866b8860eeb24cc4a345d32cc7ce7 Nix 1.12 should fix this]}}
 === Q: Can Nix code be interpolated? ===
-Yes, but not everywhere
+No, only attribute names can.
   <syntaxHighlight lang=nix>
 nix-repl> let ${"x"} = 2; in x
@@ Line 213: / Line 286: @@
 === Q: Can it be <code>eval</code>-ed from string? ===
-A: Sure, but only via "import from derivation":
+A: Yes, but it is not recommended as "eval" is generally regarded as an easy to abuse language feature. It is possible but only via the store (not as bad as "import from derivation", but still not suitable for hot code paths):
   <syntaxHighlight lang=nix>
 nix-repl> let code = "(x: x) ''id function was called''"; in import (builtins.toFile "eval" code)
 "id function was called"</syntaxHighlight>
+= Resources =
+* [https://md.darmstadt.ccc.de/xtNP7JuIQ5iNW1FjuhUccw# A separately maintained list of Nix language quirks]
 [[Category:Nix Language]]