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m1p: Map interpolation and DIY i18n++

m1p (short for "map" in the tradition of i18n) is a map interpolation library that can be used for i18n (or just externalizing textual content for a single language), theming, and similar use cases. Some assembly required. Batteries not included. Bring your own bling.

Install

With tools.deps:

no.cjohansen/m1p {:mvn/version "2023.11.07"}

With Leiningen:

[no.cjohansen/m1p "2023.11.07"]

m1p's core value proposition

m1p makes it possible to loosely couple data processing code from textual content by placing it in a dictionary, and to loosely couple its content from app data that will eventually be interpolated into it.

You produce data with placeholders for text, theming, and other "flavor content".
m1p looks up content templates from dictionaries and inflates them with app data.
m1p folds inflated content templates into your data.
Et voila!

m1p was created with these goals:

i18n, theming, and similar concerns should be handled orthogonal to data processing.
Dictionaries should be serializable data.
Dictionaries should support declarative interpolation and custom transformations.

i18n with m1p

The best way to understand m1p (pronounced "meep", or "meep, meep" if you're really feeling it) is through an example. In many ways, m1p is an i18n library that isn't really an i18n library. It's very good at retrieving and "just-in-time processing" values, but it knows nothing about locales, pluralization, number formatting, and other i18n concerns. It can, however, learn those things. We'll explore how m1p works by using it as a i18n library.

Base case

m1p works with dictionaries built from plain serializable maps:

(require '[m1p.core :as m1p])

(def dictionary
  {:header/title "Hello, world!"})

(m1p/interpolate
 [:div.main
  [:h1 [:i18n :header/title]]]       ;; 1
 {:dictionaries {:i18n dictionary}}) ;; 2

;;=> [:div.main [:h1 "Hello, world!"]]

[:i18n :header/title] is a reference tuple that refers to the key :header/title in the :i18n dictionary.
m1p can interpolate from multiple dictionaries at once. This example only has one dictionary, the :i18n one.

Where it gets interesting

Greeting the world is all well and good, but what if we desire a more personal greeting? Well, then we have to fold some data into the template before folding the result back into our data:

(m1p/interpolate
 [:div.main
  [:h1 [:i18n :header/title {:greetee "Internet"}]]]
 {:dictionaries {:i18n dictionary}})

To achieve this we expanded the reference tuple to pass some data: [:i18n :header/title {:greetee "Internet"}].

We'll also update the dictionary to include :greetee with [:fn/str ...] (more about this shortly):

(def dictionary
  (m1p/prepare-dictionary
   {:header/title [:fn/str "Hello, {{:greetee}}!"]}))

All in all, it looks like this:

(def dictionary
  (m1p/prepare-dictionary
   {:header/title [:fn/str "Hello, {{:greetee}}!"]}))

(m1p/interpolate
 [:div.main
  [:h1 [:i18n :header/title {:greetee "Internet"}]]]
 {:dictionaries {:i18n dictionary}})

;;=> [:div.main [:h1 "Hello, Internet!"]]

Yay!

m1p.core/prepare-dictionary helps dictionaries do interesting things to their values on retrieval: interpolate data, "pluralize" texts, format dates, apply gradients to colors - your imagination is the limit. These transformations are performed with dictionary functions.

[:fn/str "Hello, {{:greetee}}"] references the built-in dictionary function :fn/str, which performs classic mustachioed string interpolation. It is possible and encouraged to register your own custom dictionary functions.

Another one of m1p's built-in dictionary functions, :fn/get, is even simpler. It just gets parameters:

(def dictionary
  (m1p/prepare-dictionary
   {:header/title [:span "Hello, "
                   [:strong [:fn/get :greetee]]]}))

(m1p/interpolate
 [:div.main
  [:h1 [:i18n :header/title {:greetee "Internet"}]]]
 {:dictionaries {:i18n dictionary}})

;;=> [:div.main [:h1 [:span "Hello, " [:strong "Internet"]]]]

Building dictionaries

m1p.core/prepare-dictionary enables the use of dictionary functions, and front loads some processing for faster retrieval.

(require '[m1p.core :as m1p])

(def en-dictionary
  (m1p/prepare-dictionary
   [#:home
    {:title "Home page"                           ;; 1
     :text [:fn/str "Welcome {{:display-name}}"]} ;; 2

    #:login
    {:title "Log in"
     :help-text [:span {}                         ;; 3
                 "Need help? "
                 [:a {:href [:fn/get :url]}       ;; 4
                  "Go here"]]}]))

A dictionary can be specified as a vector of maps, in which case m1p will (apply merge ,,,) them. This makes it easy to use Clojure's namespace maps for a pleasant dictionary editing experience, and is very useful when combining dictionaries for several namespaces.
The :fn/str is a built-in dictionary function. It can be overridden.
Dictionary keys can contain any value.
Dictionary functions can be used anywhere in data structures.

Custom dictionary functions

Dictionary functions can bring any number of new features to m1p dictionaries. These functions are called with:

opt the map of options passed to m1p.core/interpolate.
params - the data passed to the reference tuple looking up the dictionary key.
Remaining values from the dictionary tuple referencing the dictionary function.

We will now consider two examples that are commonly used in i18n tooling.

Date formatters

Dates are typically formatted differently under different locales, and here's how to do it with dictionary functions:

(import '[java.time LocalDateTime]
        '[java.time.format DateTimeFormatter]
        '[java.util Locale])

(defn format-date [opt params pattern local-date]      ;; 1
  (when local-date
    (-> (DateTimeFormatter/ofPattern pattern)
        (.withLocale (Locale. (:locale opt)))          ;; 2
        (.format local-date))))

(def dictionary
  (m1p/prepare-dictionary
   {:updated-at [:fn/str "Last updated "                ;; 3
                 [:fn/date "E MMM d" [:fn/get :date]]]} ;; 4
   {:dictionary-fns {:fn/date format-date}}))           ;; 5

(m1p/interpolate
 {:text [:i18n :updated-at
         {:date (LocalDateTime/of 2023 6 8 9 37 12)}]}
 {:locale "en"                                          ;; 6
  :dictionaries {:i18n dictionary}})

;;=> {:text "Last updated Wed Jun 8"}

format-date uses Java libraries to format a local date. For ClojureScript there is goog.i18n.DateTimeFormat, which works in a very similar way.
opt is passed from the call to m1p.core/interpolate and is a convenient way to pass in things like the current locale.
Dictionary functions can be arbitrarily nested
The :updated-at key uses :fn/get to pass the date to the formatter.
"Install" the date formatter dictionary function.
Options passed to interpolate are available in dictionary functions.

Because the date argument is extracted from params with :fn/get, it must be named in the reference tuple. When dictionary keys only refer to a single value, you can use the dictionary function :fn/param to select the single argument instead:

(def dictionary
  (m1p/prepare-dictionary
   {:updated-at [:fn/str "Last updated "
                 [:fn/date "E MMM d" [:fn/param]]]
    :created-at [:fn/str "Created by {{:creator}} "
                 [:fn/date "E MMM d" [:fn/get :date]]]}
   {:dictionary-fns {:fn/date format-date}}))

(m1p/interpolate
 {:created [:i18n :created-at {:creator "Christian"
                               :date (LocalDateTime/of 2023 6 8 8 37 12)}]
  :updated [:i18n :updated-at (LocalDateTime/of 2023 6 8 9 37 12)]}
 {:locale "en"
  :dictionaries {:i18n dictionary}})

;;=>
;; {:created "Created by Christian Wed Jun 8"
;;  :updated "Last updated Wed Jun 8"}

Pluralization

Pluralization is a hard problem to solve properly across all languages, but usually a trivial matter to implement for the handful of languages a specific app supports.

Here's how to add a naive "0, 1, many"-style pluralization helper to a dictionary:

(require '[m1p.core :as m1p])

(defn pluralize [opt n & plurals]
  (-> (nth plurals (min (if (number? n) n 0) (dec (count plurals))))
      (m1p/interpolate-string {:n n} opt)))

(def dictionary
  (m1p/prepare-dictionary
   {:songs [:fn/plural "no songs" "one song" "a couple of songs" "{{:n}} songs"]}
   {:dictionary-fns {:fn/plural pluralize}}))

(m1p/interpolate
 [:ul
  [:li [:i18n :songs 0]]
  [:li [:i18n :songs 1]]
  [:li [:i18n :songs 2]]
  [:li [:i18n :songs 4]]]
 {:dictionaries {:i18n dictionary}})

;;=>
;; [:ul
;;  [:li "no songs"]
;;  [:li "one song"]
;;  [:li "a couple of songs"]
;;  [:li "4 songs"]]

But what about multiple languages?

Seems a little weird for an "i18n library" to not touch on how to switch between different languages and locales, no? Turns out switching between dictionaries isn't the hard part: just check the current locale and select the right dictionary to pass to interpolate:

(def dictionary-opts
  {:dictionary-fns {:fn/date format-date
                    :fn/plural pluralize}})

(def dictionaries
  {:en (m1p/prepare-dictionary
        {:title [:fn/str "Hello {{:display-name}}!"]}
        dictionary-opts)

   :nb (m1p/prepare-dictionary
        {:title [:fn/str "Hei {{:display-name}}!"]}
        dictionary-opts)})

(def locale :nb)

(m1p/interpolate
 [:i18n :title {:display-name "Meep meep"}]
 {:dictionaries {:i18n (get dictionaries locale)}})

;;=> "Hei Meep meep!"

Parity tests

Because dictionaries will be used interchangeably, it is a good idea to test them for parity. The m1p.validation namespace contains several functions that can detect common problems. You can combine these however you want, and possibly add some of your own and perform assertions on them in your test suite, during builds, or similar.

(require '[m1p.core :as m1p]
         '[m1p.validation :as v])

(def dicts
  {:en (m1p/prepare-dictionary
        [#:home
         {:title "Home page"
          :text [:fn/str "Welcome {{:display-name}}"]}

         #:login
         {:title "Log in"
          :help-text [:span {}
                      "Need help? "
                      [:a {:href [:fn/get :url]}
                       "Go here"]]}])

   :nb (m1p/prepare-dictionary
        [#:home
         {:title "Hjemmeside"
          :text "Welcome {{:display-name}}"} ;; Missing [:fn/str ,,,]

         #:login
         {:title "Logg inn"}])})

(concat
 (v/find-non-kw-keys dicts)
 (v/find-unqualified-keys dicts)
 (v/find-missing-keys dicts)
 (v/find-misplaced-interpolations dicts)
 (v/find-type-discrepancies dicts)
 (v/find-interpolation-discrepancies dicts)
 (v/find-fn-get-param-discrepancies dicts))

;;=>
;; [{:kind :missing-key
;;   :dictionary :nb
;;   :key :login/help-text}
;;  {:kind :misplaced-interpolation-syntax
;;   :dictionary :nb
;;   :key :home/text}
;;  {:kind :interpolation-discrepancy
;;   :key :home/text
;;   :dictionaries {:en #{:display-name}
;;                  :nb #{}}}]

All the validation functions return a list of potential problems in your dictionaries. The data can be used to generate warnings and/or errors as you see fit. For a more human consumable report, pass the data to m1p.validation/print-report (which formats the data with m1p.validation/format-report and prints it):

(->> (concat
      (v/find-non-kw-keys dicts)
      (v/find-unqualified-keys dicts)
      (v/find-missing-keys dicts)
      (v/find-misplaced-interpolations dicts)
      (v/find-type-discrepancies dicts)
      (v/find-interpolation-discrepancies dicts)
      (v/find-fn-get-param-discrepancies dicts))
     (v/print-report dicts))

;; Problems in :nb
;;   String interpolation syntax outside :fn/str:
;;     :home/text
;;
;;   Missing keys:
;;     :login/help-text
;;
;; Interpolation discrepancies
;;   :home/text
;;     :en #{:display-name}
;;     :nb #{}

Reference

Dictionary

A dictionary is just a map. One of the core ideas in m1p is to create this map in such a way that its source can be serializable Clojure data. For runtime use, pass it through m1p.core/prepare-dictionary which turns some values into functions.

Because serializable dictionaries is an important goal for m1p, the dictionary can contain reference tuples that turn into function calls on retrieval.

m1p does not have anything akin to get-in, so dictionaries is just one flat key-value data structure. For this reason, it is highly suggested to use namespaced keys in dictionaries.

Reference tuple

A reference tuple is a vector where the first element is a keyword that has meaning to m1p:

[:some-keyword ,,,]

There are two main forms of reference tuples:

References to dictionary keys in data passed to m1p.core/interpolate
References to dictionary functions in dictionary values.

Dictionary lookups

Dictionary key lookups look like this:

[dictionary-k k & [params]]

That is:

dictionary-k - A key referencing a dictionary
k - A key in said dictionary
params - Optional argument to dictionary functions

Dictionary key lookups will be replaced with the result of:

(m1p.core/lookup opt dictionary k params)

Where does the dictionary-k key come from? When calling m1p.core/interpolate, dictionaries are passed like so:

(m1p.core/interpolate params {:dictionaries {k dict}})

The k is the value to use in lookup references in params to lookup keys in that dictionary.

Dictionary function references

References to dictionary functions look like this:

[fn-k & args]

Available fn-ks are determined when calling m1p.core/prepare-dictionary:

(m1p/prepare-dictionary
 dictionary-map

 {:dictionary-fns {k fn}})

Any k in the :dictionary-fns map can be used in reference tuples in the dictionary to have the associated function called on retrieval, in addition to built-in dictionary functions, see below.

Dictionary functions

A dictionary function is a function that can be invoked on retrieval based on declarative data in the dictionary. Dictionary functions are passed to m1p.core/prepare-dictionary in the second argument under :dictionary-functions:

(m1p/prepare-dictionary
 dictionary-map

 {:dictionary-fns {k fn}})

See reference tuples for more information about referencing these in dictionaries.

Dictionary functions are called with opts from the call to m1p.core/interpolate, params from the dictionary key lookup and any remaining items from the reference tuple that invoked the function.

This reference lookup:

(def dictionary
  (m1p.core/prepare-dictionary
   {:songs [:fn/plural "no songs" "one song" "{{:n}} songs"]}
   {:dictionary-fns {:fn/plural pluralize}}))

And this interpolation:

(m1p.core/interpolate
  [:i18n :songs 0]
  {:dictionaries {:i18n dictionary}
   :fn.str/on-missing-interpolation (fn [_ _ k] (str "No" k "!"))})

Will result in pluralize being called with the options map passed to interpolate, 0 as params, and then the strings "no songs", "one song", and "{{:n}} songs". In fewer words:

(def opt {:fn.str/on-missing-interpolation (fn [_ _ k] (str "No" k "!"))})

(pluralize opt 0 "no songs" "one song" "{{:n}} songs")

`[:fn/str & xs]`

A built-in dictionary function. Performs string interpolation on the strings xs and joins the strings with no separator. Any occurrence of {{:k}} will be replaced with :k from params.

If a string contains an interpolation placeholder that is not provided in params on retrieval, the behavior is defined by the function passed as :fn.str/on-missing-interpolation in the options map to m1p.core/prepare-dictionary. This function is called with an options map, params, and the missing placeholder/key.

By default missing interpolations will render the string:

"[Missing interpolation key :k]"

You might want to provide a custom function for this to throw exceptions during test and developent, and to log the problem and output an empty string in production.

`[:fn/get k]`

Returns the key k in params in [:i18n :dict/key params]. Return the string "[Missing key k]" if not found. Change this behavior by passing a function as :fn.get/on-missing-key in the options map to m1p.core/prepare-dictionary. The function will be called with an options map, params and the missing key.

You might want to provide a custom function for this to throw exceptions during test and developent, and to log the problem in production.

`[:fn/param]`

Returns the entire params as passed to interpolate in place.

`(m1p.core/prepare-dictionary dictionary opt)`

Enables the use of dictionary functions in dictionary. opt is a map of options:

:exception-handler a function to call when a dictionary function throws an exception. The handler receives an ex-info with ex-data in this shape:
```
```clj
{:fn, :lookup-key, :data}
```
```
The handler function should return the value to interpolate in place of the failed call. Popular option: Logging the error and returning nil.

`(m1p.core/interpolate data opt)`

Interpolate data with keys from :dictionaries in opt.

Supported options:

:dictionaries the dictionaries to interpolate from
:on-missing-dictionary-key a function to call when attempting to interpolate a dictionary key that can't be found. Will be called with opts from interpolate, params, and the key.

Additional options in opt are passed to dictionary functions.

`(m1p.core/lookup opt dictionary k & [params])`

Lookup a single key k from the dictionary.

Supported options in opt:

:on-missing-dictionary-key a function to call when attempting to interpolate a dictionary key that can't be found. Will be called with opts from interpolate, params, and the key.

Additional options in opt are passed to dictionary functions.

Dictionary validation

Functions that finds common problems in dictionaries that are to be used interchangeably. In all these functions, dicts is a map of dictionaries to be compared, e.g. {:en dict, :nb dict}. All functions return a list of problems as maps with the keys :dictionary (e.g. :en), :key, :kind (the kind of problem), and optionally :data. Feel free to create your own validation functions that work on the same data structures.

(defmethod m1p.validation/get-label :my-custom-problem [_] "Custom problem")

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