L0

An experimental Fault-Tolerant Parser for the experimental L0 language. It operates as follows:

• The input text is parsed into a list of blocks. These are strings augmented by an integer which gives the indentation of a block.

• The list of blocks is transformed to a tree of blocks, where the tree structure corresponds to the level of indentation.

• An expression parser is mapped over the tree, returning a tree of "Expression blocks." The expression parser operates by first tokenizing the block content. The tokens are consumed in by a state machine that operates a stack of tokens that is reduced to an expression and whenever it is legal to do so. A reduced token sequence is an expression that is pushed onto the output stream. An error occurs when the stack is non-empty when the end of input occurs. When this happens, an error handling strategy is invoked. The result to produce a valid AST in all cases.

It is not hard to design a renderer to take advantage of the resulting AST in such way that errors are signaled in an unobrusive way, e.g. by indicating a leading bracket that has no matching right bracket in red. The goals in all cases are that

• The full text be always parsed
• Errors be signaled in place in the rendered in a discreet and helpful way.

Here is a demo.