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docs added for elvis and null coalescing operators
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# `??` Null Coalescing Operator | ||
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||
> Returns the right operand if the left operand is empty / undefined | ||
## Description | ||
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||
The null coalescing operator is a little like a conditional where the result of the | ||
operation is the first non-empty value from left to right. | ||
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||
An empty value is any of the following: | ||
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||
* an unset / undefined variable | ||
* any value with a `null` data type | ||
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||
Other "falsy" values such as numerical values of `0`, boolean `false`, zero | ||
length strings and strings containing `"null"` are not considered empty by the | ||
null coalescing operator. | ||
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||
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||
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## Examples | ||
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**Assign a variable with a default value:** | ||
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``` | ||
» $foo = $bar ?? "baz" | ||
``` | ||
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If `$bar` is unset then the value of `$foo` will be **"baz"**. | ||
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**Multiple operators:** | ||
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``` | ||
» $unset_variable ?? null ?? "foobar" | ||
foobar | ||
``` | ||
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## Detail | ||
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The following extract was taken from [Wikipedia](https://en.wikipedia.org/wiki/Null_coalescing_operator): | ||
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> The null coalescing operator (called the Logical Defined-Or operator in Perl) | ||
> is a binary operator that is part of the syntax for a basic conditional | ||
> expression in several programming languages, including C#, PowerShell as of | ||
> version 7.0.0, Perl as of version 5.10, Swift, and PHP 7.0.0. While its | ||
> behavior differs between implementations, the null coalescing operator | ||
> generally returns the result of its left-most operand if it exists and is not | ||
> null, and otherwise returns the right-most operand. This behavior allows a | ||
> default value to be defined for cases where a more specific value is not | ||
> available. | ||
> | ||
> In contrast to the ternary conditional if operator used as `x ? x : y`, but | ||
> like the binary Elvis operator used as `x ?: y`, the null coalescing operator | ||
> is a binary operator and thus evaluates its operands at most once, which is | ||
> significant if the evaluation of `x` has side-effects. | ||
## See Also | ||
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* [Pipeline](../user-guide/pipeline.md): | ||
Overview of what a "pipeline" is | ||
* [Schedulers](../user-guide/schedulers.md): | ||
Overview of the different schedulers (or 'run modes') in Murex | ||
* [`&&` And Logical Operator](../parser/logical-and.md): | ||
Continues next operation if previous operation passes | ||
* [`?:` Elvis Operator](../parser/elvis.md): | ||
Returns the right operand if the left operand is falsy | ||
* [`?` STDERR Pipe](../parser/pipe-err.md): | ||
Pipes STDERR from the left hand command to STDIN of the right hand command | ||
* [`err`](../commands/err.md): | ||
Print a line to the STDERR | ||
* [`out`](../commands/out.md): | ||
Print a string to the STDOUT with a trailing new line character | ||
* [`try`](../commands/try.md): | ||
Handles errors inside a block of code | ||
* [`trypipe`](../commands/trypipe.md): | ||
Checks state of each function in a pipeline and exits block on error | ||
* [`||` Or Logical Operator](../parser/logical-or.md): | ||
Continues next operation only if previous operation fails | ||
* [null](../commands/devnull.md): | ||
null function. Similar to /dev/null | ||
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<hr/> | ||
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This document was generated from [gen/parser/null_coalescing_op_doc.yaml](https://github.com/lmorg/murex/blob/master/gen/parser/null_coalescing_op_doc.yaml). |
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- DocumentID: null-coalescing | ||
Title: >- | ||
`??` Null Coalescing Operator | ||
CategoryID: parser | ||
Summary: >- | ||
Returns the right operand if the left operand is empty / undefined | ||
Description: |- | ||
The null coalescing operator is a little like a conditional where the result of the | ||
operation is the first non-empty value from left to right. | ||
An empty value is any of the following: | ||
* an unset / undefined variable | ||
* any value with a `null` data type | ||
Other "falsy" values such as numerical values of `0`, boolean `false`, zero | ||
length strings and strings containing `"null"` are not considered empty by the | ||
null coalescing operator. | ||
Examples: |- | ||
**Assign a variable with a default value:** | ||
``` | ||
» $foo = $bar ?? "baz" | ||
``` | ||
If `$bar` is unset then the value of `$foo` will be **"baz"**. | ||
**Multiple operators:** | ||
``` | ||
» $unset_variable ?? null ?? "foobar" | ||
foobar | ||
``` | ||
Detail: |- | ||
The following extract was taken from [Wikipedia](https://en.wikipedia.org/wiki/Null_coalescing_operator): | ||
> The null coalescing operator (called the Logical Defined-Or operator in Perl) | ||
> is a binary operator that is part of the syntax for a basic conditional | ||
> expression in several programming languages, including C#, PowerShell as of | ||
> version 7.0.0, Perl as of version 5.10, Swift, and PHP 7.0.0. While its | ||
> behavior differs between implementations, the null coalescing operator | ||
> generally returns the result of its left-most operand if it exists and is not | ||
> null, and otherwise returns the right-most operand. This behavior allows a | ||
> default value to be defined for cases where a more specific value is not | ||
> available. | ||
> | ||
> In contrast to the ternary conditional if operator used as `x ? x : y`, but | ||
> like the binary Elvis operator used as `x ?: y`, the null coalescing operator | ||
> is a binary operator and thus evaluates its operands at most once, which is | ||
> significant if the evaluation of `x` has side-effects. | ||
Related: | ||
- elvis | ||
- pipe-err | ||
- pipeline | ||
- schedulers | ||
- out | ||
- err | ||
- try | ||
- trypipe | ||
- logical-and | ||
- logical-or | ||
- "null" |
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