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Improve type inference for functions like fold
When calling a fold with an accumulator like `Nil` or `List()` one used to have add an explicit type ascription. This is now no longer necessary. When instantiating type variables that occur invariantly in the expected type of a lambda, we now replace covariant occurrences of `Nothing` in the (possibly widened) type of the accumulator with fresh type variables. The idea is that a fresh type variable in such places is always better than Nothing. For module values such as `Nil` we widen to `List[<fresh var>]`. This does possibly cause a new type error if the fold really wanted a `Nil` instance. But that case seems very rare, so it looks like a good bet in general to do the widening.
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| Original file line number | Diff line number | Diff line change |
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| object Test: | ||
| extension [A](xs: List[A]) | ||
| def foldl[B](acc: B)(f: (A, B) => B): B = ??? | ||
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| val xs = List(1, 2, 3) | ||
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| val _ = xs.foldl(List())((y, ys) => y :: ys) | ||
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| val _ = xs.foldl(Nil)((y, ys) => y :: ys) | ||
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| def partition[a](xs: List[a], pred: a => Boolean): Tuple2[List[a], List[a]] = { | ||
| xs.foldRight/*[Tuple2[List[a], List[a]]]*/((List(), List())) { | ||
| (x, p) => if (pred (x)) (x :: p._1, p._2) else (p._1, x :: p._2) | ||
| } | ||
| } | ||
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| def snoc[A](xs: List[A], x: A) = x :: xs | ||
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| def reverse[A](xs: List[A]) = | ||
| xs.foldLeft(Nil)(snoc) | ||
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| def reverse2[A](xs: List[A]) = | ||
| xs.foldLeft(List())(snoc) | ||
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| val ys: Seq[Int] = xs | ||
| ys.foldLeft(Seq())((ys, y) => y +: ys) | ||
| ys.foldLeft(Nil)((ys, y) => y +: ys) | ||
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| def dup[A](xs: List[A]) = | ||
| xs.foldRight(Nil)((x, xs) => x :: x :: xs) | ||
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| def toSet[A](xs: Seq[A]) = | ||
| xs.foldLeft(Set.empty)(_ + _) |