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Fractl supports a REPL environment. REPL is the language shell in which users can evaluate Fractl definitions and dataflow patterns. The REPL can accelerate development by allowing users to explore and fine-tune small snippets of code, instead of wrangling with the entire stack.

Getting Started

For this tutorial, we will use the Blog application from the Quick Start guide. Use the following commands to launch the REPL from the blog model directory:

cd ~/fractl-models/blog
fractl repl

This will land you in the prompt:


From this prompt you can type-in and evaluate Fractl patterns and have the results printed back. For instance, the following session shows how to create a user and add a new blog-post under that user. (The response from the Fractl evaluator is shown as comments, some responses are left-out):

blog> {:User
{:Email ""
:FirstName "Mat"
:LastName "K"}}

; ({:type-*-tag-*- :entity,
; :-*-type-*- [:Blog.Core :User],
; :Email "",
; :FirstName "Mat",
; :LastName "K",
; :MemberSince "2023-11-03T14:39:38.667403411"})

blog> {:User
{:Email? ""}
:as [:U]}
; response will be same as above

blog> {:BlogPost
{:Name "post01"
:Title "My first post"
:Content "hello, world"}
:-> [[:PostsBy :U]]}

; {:type-*-tag-*- :entity,
; :-*-type-*- [:Blog.Core :BlogPost],
; :Name "post01",
; :Title "My first post",
; :Content "hello, world",
; :__path__
; "path://Blog.Core$User/$PostsBy/Blog.Core$BlogPost/post01",
; :Id "7df7edcb-9f81-4e0d-85a9-7af62592db9d",
; :PostedOn "2023-11-03T14:40:37.583122517"}

You can type in ? at the prompt to get some useful information about the Fractl runtime - like its version, the names of the loaded components etc. Type :quit to exit the REPL.

You can start the REPL from any directory, if you set the FRACTL_MODEL_PATHS environment variable to point to the root directories that contain Fractl models.

cd /home/me/work
export FRACTL_MODEL_PATHS=/home/me/fractl-models
fractl repl blog

By default the REPL runs without logging support. Only relevant error or warning messages are printed to the standard-output. To enable full logging, start the REPL with the :with-logs option:

fractl repl :with-logs blog

A Complete Live Session

Let's see how we can use the REPL to interactively develop a complete Fractl application. As this is an illustration, the application is a really simple one - a personal TODO-list manager. The app allows to create TODO-list entries, update their status and also keep notes attached to the todo-items. Let's fire-up the REPL and create the model!

cd ~/fractl-models
fractl repl

As we are not running from a particular model directory, you should see the following generic-prompt:


The first step is to define the component:

fractl> (component :Todo.Core)
; :Todo.Core

Notice how the prompt has changed to the newly defined model-name. Now let's create an entity to represent the todo-entry:

todo> (entity :Task
{:Id {:type :Int :guid true}
:Title :String
:Created :Now
:Status {:oneof ["in-progress" "done"]
:default "in-progress"}})

Create a couple of tasks:

todo> {:Task {:Id 1 :Title "buy groceries"}}
todo> {:Task {:Id 2 :Title "prepare sales presentation"}}

List pending tasks:

todo> {:Task {:Status? "in-progress"}}

It'll be usefull to have dataflows to list pending tasks and also to mark tasks as done:

todo> (dataflow :ListPending
{:Task {:Status? "in-progress"}})
todo> (dataflow :MarkDone
{:Task {:Id? :MarkDone.Task :Status "done"}})

Mark the first task as done:

todo> {:MarkDone {:Task 1}}

A facility to manage notes under each task:

todo> (entity :Note
{:No {:type :Int :id true}
:Content :String})
todo> (relationship :Notes
{:meta {:contains [:Task :Note]}})

Add a note to the second task:

todo> {:Task {:Id? 2} :as [:T]}
todo> {:Note {:No 1 :Content "to be completed this week"}
:-> [[:Notes :T]]}

Now the model is complete and tested - you can generate a model that you can build an deploy using the dump command:

(dump :Todo.Core)

You'll find the model files under the todo directory. You can use the REPL to make changes to the component or add new components to the model - just call dump to persist your change to the file-system.