(ns introduction
  (:require [sg.flybot.pullable :as pull :refer [qfn]])) ;=> nil

Lasagna-Pull: Query Data from Deep Data Structures

The Problem

Let's see some data copied from deps.edn:

(def data {:deps
           {:paths ["src"],
            :deps {},
            :aliases
            {:dev
             {:extra-paths ["dev"],
              :extra-deps
              {'metosin/malli {:mvn/version "0.10.2"},
               'com.bhauman/figwheel-main {:mvn/version "0.2.18"}}},
             :test
             {:extra-paths ["test"],
              :extra-deps
              {'lambdaisland/kaocha {:mvn/version "1.80.1274"}},
              :main-opts ["-m" "kaocha.runner"]}}}}) ;=> #'introduction/data

In clojure.core we have get and get-in to extract information from a map, and we are very familiar with them. However, if we need to extract multiple pieces of information from different locations of the map, things are starting to get tricky. We have to call get/get-in multiple times manually.

(let [global-path     (get-in data [:deps :paths])
      dev-extra-path  (get-in data [:deps :aliases :dev :extra-paths])
      test-extra-path (get-in data [:deps :aliases :test :extra-paths])]
  (concat global-path dev-extra-path test-extra-path)) ;=> ("src" "dev" "test")

Lasagna-Pull way

Using lasagna-pull, we can do it in a more intuitive way, pull/qfn returns a query function, which matches data using a Lasagna pattern.

((qfn '{:deps {:paths   ?global
               :aliases {:dev  {:extra-paths ?dev}
                         :test {:extra-paths ?test}}}}
      (concat ?global ?dev ?test))
 data) ;=> ("src" "dev" "test")

A lasagna pattern mimics your data; you specify the pieces of information that you are interested in by using logic variables, which are just symbols starting with a ?.

💡 qfn scans your pattern, binding them, then you can use them directly in the function's body.

Another frequent situation is selecting a subset of data. We can use select-keys to shrink a map, and map it over a sequence of maps. Lasagna-pull provides this with an implicit binding &?:

((qfn '{:deps {:paths ? :aliases {:dev {:extra-paths ?}}}} &?)
 data) ;=> {:deps {:paths ["src"], :aliases {:dev {:extra-paths ["dev"]}}}}

The unnamed binding ? in the pattern is a placeholder, it will appear in &?.

Sequence of maps

The pattern to select inside the sequence of the maps just look like the data itself:

(def person&fruits {:person/name "Alan",
                    :person/age 20,
                    :person/sex :male
                    :fruits [{:name "Apple", :in-stock 10}
                             {:name "Orange", :in-stock 0}]
                    :likes  [{:person-name "Alan" :fruit-name "Apple"}]}) ;=> #'introduction/person&fruits
((qfn '{:fruits [{:name ?}]} &?)
 person&fruits) ;=> {:fruits [{:name "Apple"} {:name "Orange"}]}

By appending a logical variable to the sequence marking vector, we can capture a sequence of maps.

((qfn '{:fruits [{:name ?} ?names]} (map :name ?names))
 person&fruits) ;=> ("Apple" "Orange")

Filter a value

Sometimes, we need to filter a map on some keys. It can feel very intuitive to specify it in our pattern; let's find Alan's age.

((qfn '{:person/name "Alan", :person/age ?age} ?age)
 person&fruits) ;=> 20

The success of the above query is due to the fact that we filtered on :person/name.

Using same named lvar multiple times to join

If a named lvar bound more than one time, its value has to be the same, otherwise all matching fails. We can use this to join:

(def q2 (qfn '{:a ?x :b {:c ?c :d ?x}} [?c ?x])) ;=> #'introduction/q2
(q2 {:a 1 :b {:c 5 :d 1}}) ;=> [5 1]
(q2 {:a 1 :b {:c 5 :d 2}}) ;=> [nil nil]

The second query above failed because ?x bound twice and they do not agree with each other.

Pattern Options

In addition to basic queries, Lasagna-pull supports pattern options, which are decorators on the key part of a pattern, allowing you to fine-tune the query. An option is a pair, a keyword followed by its arguments.

General options

These options do not require a value of a certain type.

:not-found option

The easiest option is :not-found, it provides a default value if a value is not found.

((qfn '{(:a :not-found 0) ?a} ?a)
 {:b 3}) ;=> 0

:when option

:when option has a predict function as it argument, a match only succeed when this predict fulfilled by the value.

(def q (qfn {(list :a :when odd?) '?a} ?a)) ;=> #'introduction/q
(q {:a 3 :b 3}) ;=> 3

This will failed to match, e.g. returns nil:

(q {:a 2 :b 3}) ;=> nil

Note that the basic filter can also be thought as a :when option.

You can combine options together, they will be applied by their order:

((qfn {(list :a :when even? :not-found 0) '?} &?)
 {:a -1 :b 3}) ;=> {:a 0}

Pattern options requires a value be a certain type

:seq option

If you are about to match a large sequence, you might want to use pagination. This is essential if your values are lazy (sequential) .

((qfn '{(:a :seq [5 10]) ?} &?)
 {:a (range 1000)}) ;=> {:a (5 6 7 8 9 10 11 12 13 14)}

The design of putting options in the key part of the patterns, enable us to do nested queries.

(def range-data {:a (map (fn [i] {:b i}) (range 100))}) ;=> #'introduction/range-data
((qfn '{(:a :seq [90 5]) [{:b ?} ?bs]} (map :b ?bs))
 range-data) ;=> (90 91 92 93 94)

:with and :batch options

You may store a function as a value in your map, then when querying it, you can apply arguments to it, :with enable you to do it:

(defn square [x] (* x x)) ;=> #'introduction/square
((qfn '{(:a :with [5]) ?a} ?a)
 {:a square}) ;=> 25

And :batch will apply many times on it, as if it is a sequence:

((qfn {(list :a :batch (mapv vector (range 100)) :seq [40 5]) '?a} ?a)
 {:a square}) ;=> (1600 1681 1764 1849 1936)

These options not just for convinience, if the embeded functions invoked by a query has side effects, these options could do GraphQL's mutation.

(def a (atom 0)) ;=> #'introduction/a
((qfn {(list :a :with [5]) '?a} ?a)
 {:a (fn [x] (swap! a + x))}) ;=> 5

And the atom has been changed:

@a ;=> 5

Malli schema support

Lasagna-pull has optional support for malli schemas. In Clojure, by put malli in your dependencies, it automatically checks your data pattern when you make queries to ensure that the syntax is correct.

Your query patterns not only follow the rules of pattern, it also should conform to your data schema. It is especially important when you want to expose your data to the external world.

The with-data-schema macro let you include your customized data schema (applies to your data), Lasagna-pull uses it to do a deeper check for the pattern. It tries to find if the query pattern conforms to the meaning of data schema (in terms of each value and your data structrue).

(defmacro try! [& body] `(try ~@body (catch Throwable e# #:error{:msg (ex-message e#)}))) ;=> #'introduction/try!
(def my-data-schema [:map [:a :int] [:b :keyword]]) ;=> #'introduction/my-data-schema
(try!
 (pull/with-data-schema my-data-schema
   (qfn '{:c ?c} ?c))) ;throws=>#:error{:type clojure.lang.Compiler$CompilerException, :message "Syntax error compiling at (0:0).", :data #:clojure.error{:phase :compile-syntax-check, :line 0, :column 0, :source "NO_SOURCE_PATH"}}

The above throws an exception! Because, once you specified a data schema, you only allow users to query values documented in the schema (i.e. closeness of map). Since :c is not included in the schema, querying :c is invalid.

Lasagna-pull try to find all schema problems:

(try!
 (pull/with-data-schema my-data-schema
   (qfn '{(:a :not-found "3") ?} &?))) ;throws=>#:error{:type clojure.lang.Compiler$CompilerException, :message "Syntax error compiling at (0:0).", :data #:clojure.error{:phase :compile-syntax-check, :line 0, :column 0, :source "NO_SOURCE_PATH"}}

The above code also triggers an exception, because :a is an :int as in the data schema, while you provide a :not-found pattern option value which is not an integer.

Real world usage and some history

In Flybot, we use fun-map extensively. In a typical server backend, we put everything mutable into a single map called system, including atoms, databases even web server itself. The fun-map library manages the lifecycles of these components, and injects dependencies among them.

Then, we add database queries into system, they relies on databases, so the database value (we use Datomic) or connection (if using SQL databases) will be injected by fun-maps fnk, and while developing, we can easily replace the DB connection by mocked values.

Most queries also require input from user requests, so in the ring handler (of course, in system), we merge the HTTP request to system. This merged version then will be used as the source of all possible data.

The next problem is how to let user to fetch. We were not satisfied with conventional REST style; GraphQL is good, but it adds too much unneccesary middle layers in our opinion. For example, we need to write many resolvers in order to glue it to backend data.

The first attempt made was juxt pull, I contributed to the 0.2 version and used it in some projects. It allows users to construct a data pattern (basic idea and syntax is similar to EQL) to pull data from the request -enriched system.

However, the syntax of pull does not go very well with deep and complex data structures (because the database information also translated into fields). It just returns a trimmed version of the original map, users often need to walk the returned value again to find the pieces of information by themselves.

Lasagna-pull introduces a new query pattern in order to address these problems.

vs Meander Epsilon

When we were developing Lasagna-Pull, we did not know of Meander's existence. The similarity in pattern and API is merely a coincidence.

From my limited observation, Meander is actually a term rewrite language whose concept originated from logical programming. It has a far broader range of applicable areas than just querying and matching data with patterns. Lasagna-Pull, on the other hand, is a practical effort aimed solely at data and pattern querying.

Development

• Run clojure -T:build ci.
• This project's uses a lot of rct tests.

About this README

This file is generated by notebook/introduction.clj using clerk-doc.

License

Copyright. © 2022 Flybot Pte. Ltd. Apache License 2.0, http://www.apache.org/licenses/