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Try to clarify optional parameter restrictions #2621

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Sep 14, 2020
Merged

Try to clarify optional parameter restrictions #2621

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9e4cea8
Try to clarify optional parameter restrictions
jamesderlin Sep 3, 2020
c949eb3
Update with review feedback from lrhn
jamesderlin Sep 9, 2020
b760195
Update with review feedback from kwalrath
jamesderlin Sep 14, 2020
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20 changes: 12 additions & 8 deletions src/_guides/language/language-tour.md
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Expand Up @@ -1127,22 +1127,22 @@ is sometimes referred to as _arrow_ syntax.
there, but you can use a [conditional expression](#conditional-expressions).
{{site.alert.end}}

A function can have two types of parameters: _required_ and _optional_.
The required parameters are listed first, followed by any optional parameters.
Optional parameters can be _named_ or _positional_.
### Parameters
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Thanks for adding this section head! I don't know why we didn't have it before.


A function can have any number of *required positional* parameters. These can be
followed either by *named* parameters or by *optional positional* parameters
(but not both).

{{site.alert.note}}
Some APIs — notably [Flutter][] widget constructors — use only named
parameters, even for parameters that are mandatory. See the next section for
details.
{{site.alert.end}}

### Optional parameters

Optional parameters can be either named or positional, but not both.

#### Named parameters

Named parameters are optional unless they're specifically marked as required.

When calling a function, you can specify named parameters using
<code><em>paramName</em>: <em>value</em></code>. For example:

Expand Down Expand Up @@ -1179,7 +1179,11 @@ then the analyzer reports an issue.
To use the [@required][] annotation,
depend on the [meta][] package and import `package:meta/meta.dart`.

#### Positional parameters
{% comment %}
NULLSAFE: Rewrite this section.
{% endcomment %}

#### Optional positional parameters

Wrapping a set of function parameters in `[]` marks them as optional
positional parameters:
Expand Down