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ae468441dc
Implements a null coalescing operator in painless that looks like `?:`. This form was chosen to emulate Groovy's `?:` operator. It is different in that it only coalesces null values, instead of Groovy's `?:` operator which coalesces all falsy values. I believe that makes it the same as Kotlin's `?:` operator. In other languages this operator looks like `??` (C#) and `COALESCE` (SQL) and `:-` (bash). This operator is lazy, meaning the right hand side is only evaluated at all if the left hand side is null.
239 lines
8.0 KiB
Plaintext
239 lines
8.0 KiB
Plaintext
[[modules-scripting-painless-syntax]]
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=== Painless Syntax
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experimental[The Painless scripting language is new and is still marked as experimental. The syntax or API may be changed in the future in non-backwards compatible ways if required.]
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[float]
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[[painless-types]]
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=== Variable types
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Painless supports all of https://docs.oracle.com/javase/tutorial/java/nutsandbolts/variables.html[Java's types],
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including array types, but adds some additional built-in types.
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[float]
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[[painless-def]]
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==== Def
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The dynamic type `def` serves as a placeholder for any other type. It adopts the behavior
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of whatever runtime type it represents.
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[float]
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[[painless-strings]]
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==== String
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String constants can be declared with single quotes, to avoid escaping horrors with JSON:
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[source,painless]
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---------------------------------------------------------
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def mystring = 'foo';
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---------------------------------------------------------
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[float]
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[[painless-arrays]]
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==== Arrays
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Arrays can be subscripted starting from `0` for traditional array access or with
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negative numbers to starting from the back of the array. So the following
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returns `2`.
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[source,painless]
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---------------------------------------------------------
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int[] x = new int[5];
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x[0]++;
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x[-5]++;
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return x[0];
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---------------------------------------------------------
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[float]
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[[painless-lists]]
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==== List
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Lists can be created explicitly (e.g. `new ArrayList()`) or initialized similar to Groovy:
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[source,painless]
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---------------------------------------------------------
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def list = [1,2,3];
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---------------------------------------------------------
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Lists can also be accessed similar to arrays. They support `.length` and
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subscripts, including negative subscripts to read from the back of the list:
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[source,painless]
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---------------------------------------------------------
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def list = [1,2,3];
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list[-1] = 5
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return list[0]
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---------------------------------------------------------
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[float]
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[[painless-maps]]
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==== Map
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Maps can be created explicitly (e.g. `new HashMap()`) or initialized similar to Groovy:
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[source,painless]
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---------------------------------------------------------
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def person = ['name': 'Joe', 'age': 63];
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---------------------------------------------------------
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Map keys can also be accessed as properties.
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[source,painless]
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---------------------------------------------------------
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def person = ['name': 'Joe', 'age': 63];
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person.retired = true;
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return person.name
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---------------------------------------------------------
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Map keys can also be accessed via subscript (for keys containing special characters):
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[source,painless]
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---------------------------------------------------------
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return map['something-absurd!']
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---------------------------------------------------------
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[float]
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[[painless-pattern]]
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==== Pattern
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Regular expression constants are directly supported:
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[source,painless]
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---------------------------------------------------------
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Pattern p = /[aeiou]/
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---------------------------------------------------------
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Patterns can only be created via this mechanism. This ensures fast performance, regular expressions
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are always constants and compiled efficiently a single time.
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[float]
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[[modules-scripting-painless-regex-flags]]
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==== Pattern flags
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You can define flags on patterns in Painless by adding characters after the
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trailing `/` like `/foo/i` or `/foo \w #comment/iUx`. Painless exposes all the
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flags from
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https://docs.oracle.com/javase/8/docs/api/java/util/regex/Pattern.html[Java's Pattern class]
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using these characters:
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[cols="<,<,<",options="header",]
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|=======================================================================
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| Character | Java Constant | Example
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|`c` | CANON_EQ | `'å' ==~ /å/c` (open in hex editor to see)
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|`i` | CASE_INSENSITIVE | `'A' ==~ /a/i`
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|`l` | LITERAL | `'[a]' ==~ /[a]/l`
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|`m` | MULTILINE | `'a\nb\nc' =~ /^b$/m`
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|`s` | DOTALL (aka single line) | `'a\nb\nc' =~ /.b./s`
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|`U` | UNICODE_CHARACTER_CLASS | `'Ɛ' ==~ /\\w/U`
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|`u` | UNICODE_CASE | `'Ɛ' ==~ /ɛ/iu`
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|`x` | COMMENTS (aka extended) | `'a' ==~ /a #comment/x`
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|=======================================================================
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[float]
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[[painless-deref]]
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=== Dereferences
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Like lots of languages, Painless uses `.` to reference fields and call methods:
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[source,painless]
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---------------------------------------------------------
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String foo = 'foo';
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TypeWithGetterOrPublicField bar = new TypeWithGetterOrPublicField()
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return foo.length() + bar.x
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---------------------------------------------------------
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Like Groovy, Painless uses `?.` to perform null-safe references, with the
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result being `null` if the left hand side is null:
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[source,painless]
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---------------------------------------------------------
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String foo = null;
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return foo?.length() // Returns null
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---------------------------------------------------------
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Unlike Groovy, Painless doesn't support writing to null values with this
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operator:
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[source,painless]
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---------------------------------------------------------
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TypeWithSetterOrPublicField foo = null;
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foo?.x = 'bar' // Compile error
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---------------------------------------------------------
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[float]
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[[painless-operators]]
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=== Operators
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All of Java's https://docs.oracle.com/javase/tutorial/java/nutsandbolts/operators.html[operators] are
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supported with the same precedence, promotion, and semantics.
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There are only a few minor differences and add-ons:
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* `==` behaves as Java's for numeric types, but for non-numeric types acts as https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#equals-java.lang.Object-[`Object.equals()`]
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* `===` and `!==` support exact reference comparison (e.g. `x === y`)
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* `=~` true if a portion of the text matches a pattern (e.g. `x =~ /b/`)
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* `==~` true if the entire text matches a pattern (e.g. `x ==~ /[Bb]ob/`)
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The `?:` (aka Elvis) operator coalesces null values. So `x ?: 0` is `0` if `x`
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is `null` and whatever value `x` has otherwise. It is a convenient way to write
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default values like `doc['x'].value ?: 0` which is 0 if `x` is not in the
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document being processed. It can also work with null safe dereferences to
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efficiently handle null in chains. For example,
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`doc['foo.keyword'].value?.length() ?: 0` is 0 if the document being processed
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doesn't have a `foo.keyword` field but is the length of that field if it does.
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Lastly, `?:` is lazy so the right hand side is not evaluated at all if the left
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hand side isn't null.
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NOTE: Unlike Groovy, Painless' `?:` operator only coalesces `null`, not `false`
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or http://groovy-lang.org/semantics.html#Groovy-Truth[falsy] values. Strictly
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speaking Painless' `?:` is more like Kotlin's `?:` than Groovy's `?:`.
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[float]
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[[painless-control-flow]]
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=== Control flow
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Java's https://docs.oracle.com/javase/tutorial/java/nutsandbolts/flow.html[control flow statements] are supported, with the exception
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of the `switch` statement.
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In addition to Java's `enhanced for` loop, the `for in` syntax from groovy can also be used:
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[source,painless]
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---------------------------------------------------------
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for (item : list) {
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...
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}
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---------------------------------------------------------
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[float]
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[[painless-functions]]
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=== Functions
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Functions can be declared at the beginning of the script, for example:
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[source,painless]
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---------------------------------------------------------
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boolean isNegative(def x) { x < 0 }
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...
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if (isNegative(someVar)) {
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...
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}
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---------------------------------------------------------
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[float]
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[[painless-lambda-expressions]]
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=== Lambda expressions
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Lambda expressions and method references work the same as https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html[Java's].
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[source,painless]
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---------------------------------------------------------
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list.removeIf(item -> item == 2);
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list.removeIf((int item) -> item == 2);
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list.removeIf((int item) -> { item == 2 });
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list.sort((x, y) -> x - y);
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list.sort(Integer::compare);
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---------------------------------------------------------
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Method references to functions within the script can be accomplished using `this`, e.g. `list.sort(this::mycompare)`.
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