SIP - Allow multiple implicit parameter lists

sips/pending/_posts/2016-04-21-multiple-implicit-parameter-lists.md

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+---
+layout: sip
+disqus: true
+title: SIP-NN - SIP Allow multiple implicit parameter lists
+---
+
+**By: Miles Sabin and Merlin Göttlinger**
+
+## History
+
+| Date          | Version       |
+|---------------|---------------|
+| Apr 21st 2016 | Initial Draft |
+
+## Motivation
+
+Currently, the implicit parameters of a function are limited to being placed in a single, final, parameter list of a function. This prevents later parameters from using dependant types of former ones, as this is only allowed between, but not inside a single parameter list. 
+
+This restriction also means that if you want to explicitly specify one of the implicit parameters you have to specify all of them by hand (or write `implicitly` in place of every other parameter that should be filled by the compiler).
+
+## Motivating Examples
+
+### Examples
+
+For regular parameter lists it is possible to write the following:
+{% highlight scala %}
+trait A { type B }
+
+def works(a: A)(b: a.B) = "works"
+{% endhighlight %}
+Due to the restriction of being contained in a single parameter list, implicit parameters can't work in the same way. A common workaround is to pull the type member `B` into a type parameter of a new `Aux` object and make this a type parameter of your function:
+{% highlight scala %}
+trait A { type B }
+object A { type Aux[D] = A { type B = D } }
+
+def works[B](implicit a: A.Aux[B], b: B) = "works"
+{% endhighlight %}
+While this works it introduces unnecessary complexity because our method shouldn't need to have type parameters and we shouldn't need the `Aux` object just to use the type member of `A` for implicits.
+
+With the proposed change of allowing multiple implicit parameter lists the example would again be reduced to the following concise and readable example:
+{% highlight scala %}
+trait A { type B }
+
+def worksWithImplicits(implicit a: A)(implicit b: a.B) = "pls make me work"
+{% endhighlight %}
+
+Another benefit mentioned before would be that implicit parameters, that are expected to often be filled in manually can be put in separate lists.
+{% highlight scala %}
+def now(implicit a: Int, b: String) = b * a
+now(1, implicitly)
+
+def then(implicit a: Int)(implicit b: String) = b * a
+then(1)
+{% endhighlight %}
+
+## Implementation
+
+The implementation of this change is as simple as removing the restriction of having only one parameter list in the `Parser`. A pull request (#5108) from Miles Sabin already implements this change as well as an accompanying test file. The change should not affect any existing code as every Scala statement that was valid before will still be valid after the change. There will even be binary compatibility in both directions.
+The desugaring ob context and view bounds will be added on the left of the leftmost implicit parameter list to go with the general expectation from the current implementation.
+
+## References
+
+1. [Miles' PR][1]
+
+[1]: https://github.com/scala/scala/pull/5108 "PR#5108"