forked from lix-project/lix
d8bfeda164
it's more likely for readers to find it right there. this also slightly rewords examples to make them stand out better. in the long run there probably needs to be a dedicated section on formal syntax, and better highlighting of examples.
165 lines
6.4 KiB
Markdown
165 lines
6.4 KiB
Markdown
# Operators
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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* <code>\|\|</code> *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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> *attrset* `.` *attrpath* \[ `or` *expr* \]
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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 the *expr* after `or` if provided, otherwise abort evaluation.
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An attribute path is a dot-separated list of [attribute names](./values.md#attribute-set).
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> *attrpath* = *name* [ `.` *name* ]...
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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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[String and path concatenation]: #string-and-path-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 *attrset1* and *attrset2*.
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If an attribute name is present in both, the attribute value from the latter 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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