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What are type lambdas in Scala and what are their benefits?

Asked 2012-01-05T00:38:14.133
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Sometime I stumble into the semi-mysterious notation of 

def f[T](..) = new T[({type l[A]=SomeType[A,..]})#l] {..}

in Scala blog posts, which give it a "we used that type-lambda trick" handwave.

While I have some intutition about this (we gain an anonymous type parameter A without having to pollute the definition with it?), I found no clear source describing what the type lambda trick is, and what are its benefits. Is it just syntactic sugar, or does it open some new dimensions?

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The benefits are exactly the same as those conferred by anonymous functions.

def inc(a: Int) = a + 1; List(1, 2, 3).map(inc)

List(1, 2, 3).map(a => a + 1)

An example usage, with Scalaz 7. We want to use a Functor that can map a function over the second element in a Tuple2.

type IntTuple[+A]=(Int, A)
Functor[IntTuple].map((1, 2))(a => a + 1)) // (1, 3)

Functor[({type l[a] = (Int, a)})#l].map((1, 2))(a => a + 1)) // (1, 3)

Scalaz provides some implicit conversions that can infer the type argument to Functor, so we often avoid writing these altogether. The previous line can be rewritten as:

(1, 2).map(a => a + 1) // (1, 3)

If you use IntelliJ, you can enable Settings, Code Style, Scala, Folding, Type Lambdas. This then hides the crufty parts of the syntax, and presents the more palatable:

Functor[[a]=(Int, a)].map((1, 2))(a => a + 1)) // (1, 3)

A future version of Scala might directly support such a syntax.

answered 2012-01-05T04:23:38.607

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