Pipe Expressions¶

JEP:	4
Author:	Michael Dowling
Status:	accepted
Created:	07-Dec-2013

Abstract¶

This document proposes adding support for piping expressions into subsequent expressions.

Motivation¶

The current JMESPath grammar allows for projections at various points in an expression. However, it is not currently possible to operate on the result of a projection as a list.

The following example illustrates that it is not possible to operate on the result of a projection (e.g., take the first match of a projection).

Given:

{
    "foo": {
        "a": {
            "bar": [1, 2, 3]
        },
        "b": {
            "bar": [4, 5, 6]
        }
    }
}

Expression:

foo.*.bar[0]

The result would be element 0 of each bar:

[1, 4]

With the addition of filters, we could pass the result of one expression to another, operating on the result of a projection (or any expression).

Expression:

foo.*.bar | [0]

Result:

[1, 2, 3]

Not only does this give us the ability to operate on the result of a projection, but pipe expressions can also be useful for breaking down a complex expression into smaller, easier to comprehend, parts.

Modified Grammar¶

The following modified JMESPath grammar supports piped expressions.

expression        = sub-expression / index-expression / or-expression / identifier / "*"
expression        =/ multi-select-list / multi-select-hash / pipe-expression
sub-expression    = expression "." expression
pipe-expression   = expression "|" expression
or-expression     = expression "||" expression
index-expression  = expression bracket-specifier / bracket-specifier
multi-select-list = "[" ( expression *( "," expression ) ) "]"
multi-select-hash = "{" ( keyval-expr *( "," keyval-expr ) ) "}"
keyval-expr       = identifier ":" expression
bracket-specifier = "[" (number / "*") "]" / "[]"
number            = [-]1*digit
digit             = "1" / "2" / "3" / "4" / "5" / "6" / "7" / "8" / "9" / "0"
identifier        = 1*char
identifier        =/ quote 1*(unescaped-char / escaped-quote) quote
escaped-quote     = escape quote
unescaped-char    = %x30-10FFFF
escape            = %x5C   ; Back slash: \
quote             = %x22   ; Double quote: '"'
char              = %x30-39 / ; 0-9
                    %x41-5A / ; A-Z
                    %x5F /    ; _
                    %x61-7A / ; a-z
                    %x7F-10FFFF

Note

pipe-expression has a higher precendent than the or-operator

Compliance Tests¶

[{
  "given": {
    "foo": {
      "bar": {
        "baz": "one"
      },
      "other": {
        "baz": "two"
      },
      "other2": {
        "baz": "three"
      },
      "other3": {
        "notbaz": ["a", "b", "c"]
      },
      "other4": {
        "notbaz": ["d", "e", "f"]
      }
    }
  },
  "cases": [
    {
      "expression": "foo.*.baz | [0]",
      "result": "one"
    },
    {
      "expression": "foo.*.baz | [1]",
      "result": "two"
    },
    {
      "expression": "foo.*.baz | [2]",
      "result": "three"
    },
    {
      "expression": "foo.bar.* | [0]",
      "result": "one"
    },
    {
      "expression": "foo.*.notbaz | [*]",
      "result": [["a", "b", "c"], ["d", "e", "f"]]
    },
    {
      "expression": "foo | bar",
      "result": {"baz": "one"}
    },
    {
      "expression": "foo | bar | baz",
      "result": "one"
    },
    {
      "expression": "foo|bar| baz",
      "result": "one"
    },
    {
      "expression": "not_there | [0]",
      "result": null
    },
    {
      "expression": "not_there | [0]",
      "result": null
    },
    {
      "expression": "[foo.bar, foo.other] | [0]",
      "result": {"baz": "one"}
    },
    {
      "expression": "{\"a\": foo.bar, \"b\": foo.other} | a",
      "result": {"baz": "one"}
    },
    {
      "expression": "{\"a\": foo.bar, \"b\": foo.other} | b",
      "result": {"baz": "two"}
    },
    {
      "expression": "{\"a\": foo.bar, \"b\": foo.other} | *.baz",
      "result": ["one", "two"]
    },
    {
      "expression": "foo.bam || foo.bar | baz",
      "result": "one"
    },
    {
      "expression": "foo | not_there || bar",
      "result": {"baz": "one"}
    }
  ]
}]

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