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Ohm v16.0

Upgrading​

Args to _iter and _nonterminal actions​

The _iter and _nonterminal actions now take a variable number of arguments, rather than a single Node[] argument containing the child nodes. To make existing code work with Ohm v16, you should change the parameter to a rest parameter — e.g., _iter(children) { ... } should be changed to _iter(...children) { ... }. You can easily find code that needs to change because addOperation and friends will now throw an exception if your actions have a single parameter which is not a rest parameter. See #324 for the reasons behind this change.

Default semantic actions​

In operations and attributes, if you haven't defined a semantic action for a particular rule application node, a default action will be used in some cases. For example, your grammar has an AddExp rule but your action dictionary doesn't contain a semantic action named 'AddExp'. In Ohm v16.0, there is no longer a default action for iteration nodes — it is only defined for non-terminal nodes with exactly one child. See #309 for context on this change.

Semantic actions that worked in previous versions of Ohm may need to be modified to work with v16.0. You can identify the code that needs modification because they will now raise a "missing semantic action" error. The recommended way of dealing with iteration nodes is to use the children attribute to explicitly invoke the operation on each child, e.g. iterNode.myOperation() becomes iterNode.children.map(c => c.myOperation()). This way, it's clear to readers of the code that the result is an array.

In some cases, it makes sense to write a generic _iter action that specifies the behaviour for all iteration nodes. This also makes it possible to replicate the old behaviour of the default action:

_iter(...children) {
  return children.map(c => c.myOperation());
}

grammarFromScriptElement / grammarsFromScriptElements​

The functions grammarFromScriptElement and grammarsFromScriptElements have been removed. When using Ohm in the browser, it's now recommended to put your grammar in a template literal with String.raw:

const myGrammar = ohm.grammar(String.raw`
  MyGrammar {
    greeting = "Hello" | "Hola"
  }
`);

Type parameters for operations and actions (TypeScript only)​

On semantics objects, the methods addOperation, extendOperation, addAttribute, and extendAttribute now have a required type parameter, which is the return type of the operation (or the type of the attribute). The related types Action and ActionDict are similarly parameterized by the return type of the semantic actions. In other words, all of the semantic actions for a particular operation/attribute are now required to have a consistent return type.

The simplest change to make your code compatible with Ohm v16 is to simply specify the type paramter. E.g., semantics.addOperation('toString', { ... }) becomes semantics.addOperation<string>('toString', { ... }). In some cases, it may be necessary to use any, which is the type that was assumed in previous versions of Ohm.

Other changes​

  • Grammar.semantics has been completely removed. Use Grammar.createSemantics instead.

New deprecations​

  • Node.primitiveValue is now deprecated in favour of Node.sourceString.