== Why is this an issue?

In Dart there's an ability to use template literals instead of concatenation. Since their use is clearer and more concise, they are preferred.

=== Exceptions

Concatenation of string literals, without any variables, is allowed, both using `+` and using adjacent strings. Those are typically used for multiline strings.

Raw string literals are also an exception to this rule, since they don't support interpolation.

[source,dart]
----
var s1 = 
  'hello\n' + 
  'world'; // OK
var s2 = 
  'hello\n' 
  'world'; // OK
var s3 = r'hello\n' + s1; // OK
----

The multiline strings like `s1` and `s2` above can also be written as follows:

[source,dart]
----
var s1Alternative = '''
hello
world'''; // OK
----

=== Noncompliant code example

[source,dart,diff-id=1,diff-type=noncompliant]
----
void sayHello(name) {
  print('hello ' + name + '!');  // Noncompliant
}
----

=== Compliant solution

[source,dart,diff-id=1,diff-type=compliant]
----
void sayHello(name) {
  print('hello $name!');
}
----

== Resources

=== Documentation

* Dart Docs - https://dart.dev/tools/linter-rules/prefer_interpolation_to_compose_strings[Dart Linter rule - prefer_interpolation_to_compose_strings]
* Dart Docs - https://dart.dev/language/built-in-types#strings[Dart language - strings]


ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

Use interpolation to compose strings and values.

=== Highlighting

The entire string concatenation expression, including the operands. 

If a non-parenthesized sequence of concatenations is detected, a single issue is reported, on the first part of it: e.g. in `'a' + s1 + 'a'` and `s1 + 'a' + s1`, only `'a' + s1` and `s1 + 'a'` is reported, respectively.

An exception is made for single string variables or literals in parentheses, where the outer concatenation is reported: e.g. in `'a' + (s1)`, the entire expression is reported.

In parenthesized expressions, only innermost concatenations are reported: e.g. in `'a' + (s1 + 'a')`, only `s1 + 'a'` is reported.

'''
== Comments And Links
(visible only on this page)

endif::env-github,rspecator-view[]