Deep precedence

[elegios]

The current approach to precedence lacks an important guarantee: if all operators have defined relative precedence and compatible associativity, then no expression using those operators should have ambiguous grouping. This can for example be seen in the following example:

type Exp
grouping "(" Exp ")"

syncon top: Top = e:Exp

syncon unit: Exp = "unit"
infix in: Exp = "in"
prefix pre: Exp = "pre"

precedence {
  in;
  pre;
}

The expression unit in pre unit in unit produces the following error:

Ambiguity error with 2 alternatives.

  ( unit in pre unit ) in unit
  unit in pre ( unit in unit )

 deep.test#1:1:
   1: unit in pre unit in unit

Most likely the former alternative is undesired. The issue is that the precedence error requires looking more than one level down the tree (since pre is an indirect child of the right in).