forked from syndicate-lang/preserves
92 lines
3.1 KiB
Markdown
92 lines
3.1 KiB
Markdown
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---
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no_site_title: true
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title: "Preserves Path"
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---
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Tony Garnock-Jones <tonyg@leastfixedpoint.com>
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August 2021. Version 0.1.0.
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XML documents can move into attributes, into text, or into children.
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Preserves documents don't have attributes, but they do have children
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generally and keyed children in particular. You might want to move
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into the child with a particular key (number, for sequences, or
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general-value for dictionaries); into all keys; into all
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mapped-to-values, i.e. children (n.b. not just for sequences and
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dicts, but also for sets).
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## Expressions
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Expressions: compute a sequence or set (or dictionary?) of results
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from a stream of input values.
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Precedence groupings from highest to lowest. Within a grouping, no
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mixed precedence is permitted.
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step ... ;; Applies steps one after the other, flatmap-style
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! expr ;; If no nodes, yields a dummy #t node; if some, yields none
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expr ~ expr ~ ... ;; "interleave" of expressions (sequence-valued, duplicates allowed)
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expr + expr + ... ;; "union" of expressions (set-valued)
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expr & expr & ... ;; "intersection" of expressions (set-valued)
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A step is an axis, a filter, or `[expr]`, a parenthesis for overriding precedence.
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## Axes
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Axes: move around, applying filters after moving
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.= ;; Doesn't move anywhere
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/ ;; Moves into immediate children (values / fields)
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// ;; Flattens children recursively
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. key ;; Moves into named child
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.^ ;; Moves into record label
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.keys ;; Moves into *keys* rather than values
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.length ;; Moves into the number of keys
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.annotations ;; Moves into any annotations that might be present
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.embedded ;; Moves into the representation of an embedded value
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## Filters
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Filters: narrow down a selection without moving
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=* ;; Accepts all
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=! ;; Rejects all
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= literal ;; Matches values equal to the literal
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=r regex ;; Matches strings and symbols by regular expression
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?[expr] ;; Applies the expression to each node; keeps nodes that yield nonempty
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^ literal ;; Matches a record having a the literal as its label -- equivalent to ?[.^ = literal]
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bool ;; Type filters
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float
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double
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int
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string
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bytes
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symbol
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rec
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seq
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set
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dict
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embedded
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## Transformers
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e.g. stringify results; sequenceify results (see "+" operator); setify
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results (see "/" and "&" operators); join stringified results with a
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separator
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## Tool design
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When processing multiple input documents sequentially, will sometimes
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want a list of results for each document, a set of results for each
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document, or a list flattened into a sequence of outputs for all input
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documents in the sequence. (A flattened set doesn't make sense for
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streaming since the input documents come in a sequence; if the inputs
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were treated as a set represented as a sequence, and outputs were
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buffered in a single large set, that could work out...)
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