Implicit Lifting

At its core, DaeDaLus has three semantics categories:

• semantic values (names start with a lower case letter),

• character classes (names start with $), and

• parsers (names start with an upper case letter).

Various language constructs have expectations about the kinds of entities they would be used with. For example:

• the operator + expects two semantic values;

• the construct $[ ] expects a character class;

• the construct Many expects a parser.

DaeDaLus allows more flexible expressions where language constructs and declarations may be used with arguments that do not exactly match the expected categories, subject to the following conversions:

From	Context	Effect
Value x	Char Class	[x] singleton char class
Value x	Parser	^ x succeed with the value
Char Class $x	Value	Not allowed
Char Class $x	Parser	$[ $x ] match the char class

In addition to these conversions, DaeDaLus also allows using parsers in contexts where semantic values are expected, as long as the overall expression was already a parser. For example, while + normally expects two semantic values, we also allow expressions like P + Q where + is used with two parsers as arguments.

The meaning of such expressions is that we first execute the parsers in left-to-right order, and then apply the function to their results. For example, P + Q is exactly equivalent to the following parser:

block
  let x = P
  let y = Q
  ^ (x + y)