Merge pull request #7498 from fricklerhandwerk/path+string
refactor documentation of operators, document `+` for strings and paths
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Eelco Dolstra
GitHub
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3172c51baf
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# Operators
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The table below lists the operators in the Nix language, in
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order of precedence (from strongest to weakest binding).
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| Name | Syntax | Associativity | Precedence |
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|----------------------------------------|--------------------------------------------|---------------|------------|
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| [Attribute selection] | *attrset* `.` *attrpath* \[ `or` *expr* \] | none | 1 |
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| Function application | *func* *expr* | left | 2 |
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| [Arithmetic negation][arithmetic] | `-` *number* | none | 3 |
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| [Has attribute] | *attrset* `?` *attrpath* | none | 4 |
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| List concatenation | *list* `++` *list* | right | 5 |
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| [Multiplication][arithmetic] | *number* `*` *number* | left | 6 |
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| [Division][arithmetic] | *number* `/` *number* | left | 6 |
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| [Subtraction][arithmetic] | *number* `-` *number* | left | 7 |
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| [Addition][arithmetic] | *number* `+` *number* | left | 7 |
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| [String concatenation] | *string* `+` *string* | left | 7 |
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| [Path concatenation] | *path* `+` *path* | left | 7 |
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| [Path and string concatenation] | *path* `+` *string* | left | 7 |
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| [String and path concatenation] | *string* `+` *path* | left | 7 |
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| Logical negation (`NOT`) | `!` *bool* | none | 8 |
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| [Update] | *attrset* `//` *attrset* | right | 9 |
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| [Less than][Comparison] | *expr* `<` *expr* | none | 10 |
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| [Less than or equal to][Comparison] | *expr* `<=` *expr* | none | 10 |
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| [Greater than][Comparison] | *expr* `>` *expr* | none | 10 |
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| [Greater than or equal to][Comparison] | *expr* `>=` *expr* | none | 10 |
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| [Equality] | *expr* `==` *expr* | none | 11 |
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| Inequality | *expr* `!=` *expr* | none | 11 |
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| Logical conjunction (`AND`) | *bool* `&&` *bool* | left | 12 |
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| Logical disjunction (`OR`) | *bool* `||` *bool* | left | 13 |
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| [Logical implication] | *bool* `->` *bool* | none | 14 |
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[string]: ./values.md#type-string
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[path]: ./values.md#type-path
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[number]: ./values.md#type-number
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[list]: ./values.md#list
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[attribute set]: ./values.md#attribute-set
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## Attribute selection
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Select the attribute denoted by attribute path *attrpath* from [attribute set] *attrset*.
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If the attribute doesn’t exist, return *value* if provided, otherwise abort evaluation.
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<!-- FIXME: the following should to into its own language syntax section, but that needs more work to fit in well -->
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An attribute path is a dot-separated list of attribute names.
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An attribute name can be an identifier or a string.
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> *attrpath* = *name* [ `.` *name* ]...
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> *name* = *identifier* | *string*
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> *identifier* ~ `[a-zA-Z_][a-zA-Z0-9_'-]*`
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[Attribute selection]: #attribute-selection
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## Has attribute
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> *attrset* `?` *attrpath*
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Test whether [attribute set] *attrset* contains the attribute denoted by *attrpath*.
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The result is a [Boolean] value.
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[Boolean]: ./values.md#type-boolean
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[Has attribute]: #has-attribute
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## Arithmetic
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Numbers are type-compatible:
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Pure integer operations will always return integers, whereas any operation involving at least one floating point number return a floating point number.
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See also [Comparison] and [Equality].
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The `+` operator is overloaded to also work on strings and paths.
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[arithmetic]: #arithmetic
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## String concatenation
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> *string* `+` *string*
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Concatenate two [string]s and merge their string contexts.
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[String concatenation]: #string-concatenation
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## Path concatenation
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> *path* `+` *path*
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Concatenate two [path]s.
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The result is a path.
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[Path concatenation]: #path-concatenation
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## Path and string concatenation
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> *path* + *string*
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Concatenate *[path]* with *[string]*.
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The result is a path.
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> **Note**
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>
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> The string must not have a string context that refers to a [store path].
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[Path and string concatenation]: #path-and-string-concatenation
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## String and path concatenation
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> *string* + *path*
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Concatenate *[string]* with *[path]*.
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The result is a string.
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> **Important**
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>
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> The file or directory at *path* must exist and is copied to the [store].
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> The path appears in the result as the corresponding [store path].
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[store path]: ../glossary.md#gloss-store-path
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[store]: ../glossary.md#gloss-store
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[Path and string concatenation]: #path-and-string-concatenation
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## Update
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> *attrset1* + *attrset2*
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Update [attribute set] *attrset1* with names and values from *attrset2*.
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The returned attribute set will have of all the attributes in *e1* and *e2*.
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If an attribute name is present in both, the attribute value from the former is taken.
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[Update]: #update
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## Comparison
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Comparison is
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- [arithmetic] for [number]s
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- lexicographic for [string]s and [path]s
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- item-wise lexicographic for [list]s:
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elements at the same index in both lists are compared according to their type and skipped if they are equal.
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All comparison operators are implemented in terms of `<`, and the following equivalencies hold:
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| comparison | implementation |
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|--------------|-----------------------|
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| *a* `<=` *b* | `! (` *b* `<` *a* `)` |
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| *a* `>` *b* | *b* `<` *a* |
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| *a* `>=` *b* | `! (` *a* `<` *b* `)` |
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[Comparison]: #comparison-operators
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## Equality
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- [Attribute sets][attribute set] and [list]s are compared recursively, and therefore are fully evaluated.
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- Comparison of [function]s always returns `false`.
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- Numbers are type-compatible, see [arithmetic] operators.
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- Floating point numbers only differ up to a limited precision.
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[function]: ./constructs.md#functions
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[Equality]: #equality
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## Logical implication
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Equivalent to `!`*b1* `||` *b2*.
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[Logical implication]: #logical-implication
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| Name | Syntax | Associativity | Description | Precedence |
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| ------------------------ | ----------------------------------- | ------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------- |
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| Select | *e* `.` *attrpath* \[ `or` *def* \] | none | Select attribute denoted by the attribute path *attrpath* from set *e*. (An attribute path is a dot-separated list of attribute names.) If the attribute doesn’t exist, return *def* if provided, otherwise abort evaluation. | 1 |
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| Application | *e1* *e2* | left | Call function *e1* with argument *e2*. | 2 |
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| Arithmetic Negation | `-` *e* | none | Arithmetic negation. | 3 |
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| Has Attribute | *e* `?` *attrpath* | none | Test whether set *e* contains the attribute denoted by *attrpath*; return `true` or `false`. | 4 |
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| List Concatenation | *e1* `++` *e2* | right | List concatenation. | 5 |
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| Multiplication | *e1* `*` *e2*, | left | Arithmetic multiplication. | 6 |
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| Division | *e1* `/` *e2* | left | Arithmetic division. | 6 |
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| Addition | *e1* `+` *e2* | left | Arithmetic addition. | 7 |
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| Subtraction | *e1* `-` *e2* | left | Arithmetic subtraction. | 7 |
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| String Concatenation | *string1* `+` *string2* | left | String concatenation. | 7 |
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| Not | `!` *e* | none | Boolean negation. | 8 |
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| Update | *e1* `//` *e2* | right | Return a set consisting of the attributes in *e1* and *e2* (with the latter taking precedence over the former in case of equally named attributes). | 9 |
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| Less Than | *e1* `<` *e2*, | none | Arithmetic/lexicographic comparison. | 10 |
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| Less Than or Equal To | *e1* `<=` *e2* | none | Arithmetic/lexicographic comparison. | 10 |
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| Greater Than | *e1* `>` *e2* | none | Arithmetic/lexicographic comparison. | 10 |
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| Greater Than or Equal To | *e1* `>=` *e2* | none | Arithmetic/lexicographic comparison. | 10 |
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| Equality | *e1* `==` *e2* | none | Equality. | 11 |
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| Inequality | *e1* `!=` *e2* | none | Inequality. | 11 |
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| Logical AND | *e1* `&&` *e2* | left | Logical AND. | 12 |
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| Logical OR | *e1* <code>||</code> *e2* | left | Logical OR. | 13 |
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| Logical Implication | *e1* `->` *e2* | none | Logical implication (equivalent to <code>!e1 || e2</code>). | 14 |
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@ -85,9 +85,10 @@
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Numbers, which can be *integers* (like `123`) or *floating point*
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(like `123.43` or `.27e13`).
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Numbers are type-compatible: pure integer operations will always
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return integers, whereas any operation involving at least one
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floating point number will have a floating point number as a result.
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See [arithmetic] and [comparison] operators for semantics.
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[arithmetic]: ./operators.md#arithmetic
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[comparison]: ./operators.md#comparison
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- <a id="type-path" href="#type-path">Path</a>
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