Crate nu_parser

nu-parser, the Nushell parser

Nushell’s parser is a type-directed parser, meaning that the parser will use type information available during parse time to configure the parser. This allows it to handle a broader range of techniques to handle the arguments of a command.

Nushell’s base language is whitespace-separated tokens with the command (Nushell’s term for a function) name in the head position:

head1 arg1 arg2 | head2

Lexing

The first job of the parser is to a lexical analysis to find where the tokens start and end in the input. This turns the above into:

<item: "head1">, <item: "arg1">, <item: "arg2">, <pipe>, <item: "head2">

At this point, the parser has little to no understanding of the shape of the command or how to parse its arguments.

Lite parsing

As Nushell is a language of pipelines, pipes form a key role in both separating commands from each other as well as denoting the flow of information between commands. The lite parse phase, as the name suggests, helps to group the lexed tokens into units.

The above tokens are converted the following during the lite parse phase:

Pipeline:
  Command #1:
    <item: "head1">, <item: "arg1">, <item: "arg2">
  Command #2:
    <item: "head2">

Parsing

The real magic begins to happen when the parse moves on to the parsing stage. At this point, it traverses the lite parse tree and for each command makes a decision:

• If the command looks like an internal/external command literal: e.g. foo or /usr/bin/ls, it parses it as an internal or external command
• Otherwise, it parses the command as part of a mathematical expression

Types/shapes

Each command has a shape assigned to each of the arguments it reads in. These shapes help define how the parser will handle the parse.

For example, if the command is written as:

where $x > 10

When the parsing happens, the parser will look up the where command and find its Signature. The Signature states what flags are allowed and what positional arguments are allowed (both required and optional). Each argument comes with a Shape that defines how to parse values to get that position.

In the above example, if the Signature of where said that it took three String values, the result would be:

CallInfo:
  Name: `where`
  Args:
    Expression($x), a String
    Expression(>), a String
    Expression(10), a String

Or, the Signature could state that it takes in three positional arguments: a Variable, an Operator, and a Number, which would give:

CallInfo:
  Name: `where`
  Args:
    Expression($x), a Variable
    Expression(>), an Operator
    Expression(10), a Number

Note that in this case, each would be checked at compile time to confirm that the expression has the shape requested. For example, "foo" would fail to parse as a Number.

Finally, some Shapes can consume more than one token. In the above, if the where command stated it took in a single required argument, and that the Shape of this argument was a MathExpression, then the parser would treat the remaining tokens as part of the math expression.

CallInfo:
  Name: `where`
  Args:
    MathExpression:
      Op: >
      LHS: Expression($x)
      RHS: Expression(10)

When the command runs, it will now be able to evaluate the whole math expression as a single step rather than doing any additional parsing to understand the relationship between the parameters.

Making space

As some Shapes can consume multiple tokens, it’s important that the parser allow for multiple Shapes to coexist as peacefully as possible.

The simplest way it does this is to ensure there is at least one token for each required parameter. If the Signature of the command says that it takes a MathExpression and a Number as two required arguments, then the parser will stop the math parser one token short. This allows the second Shape to consume the final token.

Another way that the parser makes space is to look for Keyword shapes in the Signature. A Keyword is a word that’s special to this command. For example in the if command, else is a keyword. When it is found in the arguments, the parser will use it as a signpost for where to make space for each Shape. The tokens leading up to the else will then feed into the parts of the Signature before the else, and the tokens following are consumed by the else and the Shapes that follow.

Structs

KnownExternal

LexState

LiteBlock

LiteCommand

Token

Enums

FlatShape

ParserPath

An abstraction over a PathBuf that can have virtual paths (files and directories). Virtual paths always exist and represent a way to ship Nushell code inside the binary without requiring paths to be present in the file system.

TokenContents

Constants

ALIASABLE_PARSER_KEYWORDS

These parser keywords can be aliased

DURATION_UNIT_GROUPS

LIB_DIRS_VAR

RESERVED_VARIABLE_NAMES

UNALIASABLE_PARSER_KEYWORDS

These parser keywords cannot be aliased (either not possible, or support not yet added)

Functions

escape_for_script_arg

find_dirs_var

find_in_dirs

This helper function is used to find files during parsing

flatten_block

flatten_expression

flatten_pipeline

flatten_pipeline_element

is_math_expression_like

is_unaliasable_parser_keyword

Check whether spans start with a parser keyword that can be aliased

lex

lex_n_tokens

Lex until the output is max_tokens longer than before the call, or until the input is exhausted. The return value indicates how many tokens the call added to / removed from the output.

lex_signature

lite_parse

parse

parse_alias

parse_attribute_block

parse_block

parse_const

Additionally returns a span encompassing the variable name, if successful.

parse_def

parse_def_predecl

parse_export_env

parse_export_in_block

parse_export_in_module

parse_expression

parse_extern

parse_external_call

parse_for

parse_hide

parse_keyword

This is a new more compact method of calling parse_xxx() functions without repeating the parse_call() in each function. Remaining keywords can be moved here.

parse_let

parse_module

parse_module_block

parse_module_file_or_dir

parse_mut

parse_overlay_hide

parse_overlay_new

parse_overlay_use

parse_source

parse_unit_value

parse_use

parse_where

parse_where_expr

trim_quotes

trim_quotes_str

unescape_unquote_string