api-builder

A Clojure library for writing APIs. API functions and vars are augmented versions of standard functions and vars. This augmentation happens at compile time and can bring features like validation of parameters based on data schemas, logging of parameters, augmentation of docstring based on parameter schemas, or even tagging of functions for later grouping in documentation.

To use it, add [com.palletops/api-builder "0.3.1"] to your :dependencies.

Motivation

When writing libraries we usually want to provide an API for them --a subset of all the functions in the library that are meant for users to use-- and we like to treat these API functions differently from the rest of (more internal) functions. For example, we want API functions to:

• check the format and type of parameters
• add the function's signature into the docstring
• add information about which errors the function can return
• ... and even be able group API functions in different groups based on domain or other concepts.

This library allows us to do that by building our own def, defn and defmulti macros for API functions with a custom set of stages that each add new properties to the functions created.

Usage

This library provides forms to create new versions of clojure's Var defining forms.

The def-defn form creates new versions of clojure's defn forms. These custom defn forms augment the created functions with extra behavior. This new behavior is defined in a pipeline of stage functions, or stages.

Similarly, def-def creates enhanced versions of clojure's def forms, and def-defmulti enhanced versions of defmulti forms.

For example, the provided defn-api macro creates functions with with error validation, signature validation, addition of signature in docstring, and a few logging concerns. This macro is defined as:

(def-defn defn-api
  [(validate-errors (constantly true))
   (validate-sig)
   (add-sig-doc)
   (log-scope)
   (log-entry)
   (log-exit)])

Once this api-defn macro is defined, we can use it do define our API functions, for example:

> (require '[com.palletops.api-builder.api :refer [defn-api]])
> (require '[schema.core :as s])

> (defn-api my-fun
   "My API fun"
   {:domain :main-api
    :sig [[s/Any :- s/Keyword]]
    :errors [{:type :example}]}
    [x]
    (if (string? x)
      (keyword x)
      (throw
        (ex-info "Can't create a keyword"
          {:type :example}))))

We can then see how the add-sig-doc stage added the signature to the docstring:

> (doc my-fun)
-------------------------
user/my-fun
([x])
  My API fun

## Function Signatures
  - Str -> Keyword

And also how the parameter checks are in place:

> (my-fun "my-key")
:my-key
> (my-fun 1)
ExceptionInfo Value does not match schema: [(named (not (instance? java.lang.String 1)) "x")]  schema.core/validate (core.clj:165)

Built in Stages

Api-builder comes with some useful pre-defined stages:

Additional metadata

The add-meta stage can be used to add constant values to a functions metadata.

Error validation

For development purposes. Helps you validate that your functions declare all the error conditions they can throw. These error conditions should be declared in the function's metadata under the :errors, and each condition is represented by a different value of the ex-data field in the ex-info exception throw by the function.

Whether this error validation takes place at runtime is controlled by the value of *validate-errors* at compile time (and also by clojure's assertion validation: *assert*). This var needs to be set to false at compile time to disable validation at runtime.

Argument Validation

The validate-sig stage validates the function's arguments and return values against the :sig metadata key that contains a list of function schemas, one for each of the function arities. Each function schema is in itself a sequence of schema as defined by prismatic/schema. The first value of each sequence is the return type, followed for a schema for each argument in the function. A vararg final arg is represented as a single schema that should match all varargs.

The validate-optional-sig stage validates the function's arguments and return values against the :sig metadata as for validate-sig, but only if the :sig metadata is specified.

Logging

The log-scope stage allows setting of log-config logging scopes for :domain, :tags, and :context.

The log-entry stage logs function entry at :trace using Timbre.

The log-exit stage logs function exit at :trace using Timbre.

Defining Custom Stages

A stage is a function that takes and a DefnMap, which is a representation of a defn form. The stage returns a modified version of the map.

License

Copyright © 2014 Hugo Duncan

Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.