Programming language: Crystal
License: MIT License
Tags: HTML Builders    
Latest version: v1.0.0

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Form Builder.cr

Dead simple HTML form builder for Crystal with built-in support for many popular UI libraries such as Bootstrap. Works well with your favourite Crystal web framework such as Kemal, Amber, or Lucky.


  • Easily generate HTML markup for forms, labels, inputs, help text and errors
  • Integrates with many UI libraries such as Bootstrap
  • Custom theme support

Supported UI Libraries

Out of the box Form Builder can generate HTML markup for the following UI libraries:

  • Bootstrap 4
    • theme: :bootstrap_4_vertical
    • theme: :bootstrap_4_inline
    • theme: :bootstrap_4_horizontal or theme: FormBuilder::Themes::Bootstrap4Horizontal.new(column_classes: ["col-sm-3","col-sm-9"])
  • Bootstrap 3
    • theme: :bootstrap_3_vertical
    • theme: :bootstrap_3_inline
    • theme: :bootstrap_3_horizontal or theme: FormBuilder::Themes::Bootstrap3Horizontal.new(column_classes: ["col-sm-3","col-sm-9"])
  • Bootstrap 2
    • theme: :bootstrap_2_vertical
    • theme: :bootstrap_2_inline
    • theme: :bootstrap_2_horizontal
  • Bulma
    • theme: :bulma_vertical
    • theme: :bulma_horizontal
  • Foundation
    • theme: :foundation
  • Materialize
    • theme: :materialize
  • Milligram
    • theme: :milligram
  • Semantic UI
    • theme: :semantic_ui_vertical
    • theme: :semantic_ui_inline
  • None (Default)
    • theme: :default
    • theme: nil
    • or simply do not provide a :theme argument

If you dont see your favourite UI library here feel free to create a PR to add it. I recommend creating an issue to discuss it first.


Add this to your application's shard.yml:

    github: westonganger/form_builder.cr
require "form_builder"


The following field types are supported:

  • :checkbox
  • :file
  • :hidden
  • :password
  • :radio
  • :select
  • :text
  • :textarea

FormBuilder in View Templates (Example in Slang)

== FormBuilder.form(theme: :bootstrap_4_vertical, action: "/products", method: :post, form_html: {style: "margin-top: 20px;", "data-foo" => "bar"}) do |f|
      ### -- Field Options
      ### type : (String | Symbol)
      ### name : (String | Symbol)?
      ### label : (String | Bool)? = true
      ### help_text : String?

      ### value : (String | Symbol)?
      ### -- Note: The `input_html["value"]` option will take precedence over the :value option (except for `type: :textarea/:select`)

      ### errors : (Array(String) | String)?
      ### -- Note: Array(String) generates a list of help text elements. If you have an Array of errors and you only want a single help text element, then join your errors array to a String

      ### -- For the following Hash options, String keys will take precedence over any Symbol keys
      ### input_html : (Hash | NamedTuple)? ### contains attributes to be added to the input/field
      ### label_html : (Hash | NamedTuple)? ### contains attributes to be added to the label
      ### wrapper_html : (Hash | NamedTuple)? ### contains attributes to be added to the outer wrapper for the label and input
      ### help_text_html : (Hash | NamedTuple)? ### contains attributes to be added to the help text container
      ### error_html : (Hash | NamedTuple)? ### contains attributes to be added to the error container(s) 

      == f.field name: "product[name]", label: "Name", type: :text, errors: product_errors["name"]

      == f.field name: "product[description]", label: "Description", type: :textarea, input_html: {class: "foobar"}, wrapper_html: {style: "margin-top: 10px"}, label_html: {style: "color: red;"}

      == f.field name: "product[file]", type: :file, help_text: "Must be a PDF", help_text_html: {style: "color: blue;"}

      == f.field name: "product[available]", type: :checkbox, label: "In Stock?"

      == f.field name: "product[class]", type: :radio, label: false

      == f.field name: "product[secret]", type: :hidden, value: "foobar"

    ### -- Additional Options for `type: :select`
    ### collection : (Hash | NamedTuple) = {
    ###   options : (Array(String) | Array(String | Array(String)) | String) ### Required, Note: The non-Array String type is for passing in a pre-built html options string
    ###   selected : (String | Array(String))?
    ###   disabled : (String | Array(String))?
    ###   include_blank : (String | Bool)?
    ### }
    ### -- Note: String keys will take precedence over any Symbol keys

    ### -- When passing a nested array to collection[:options] the Option pairs are defined as: [required_value, optional_label]
    - opts = [["A", "Type A"], ["B" "Type B"], ["C", "Type C"], "Other"]

    == f.field name: "product[type]", label: "Type", type: :select, collection: {options: opts, selected: ["B"], disabled: ["C"]}

FormBuilder in Plain Crystal Code

When using the FormBuilder.form method in plain Crystal code, the << syntax is required to add the generated field HTML to the form HTML string

form_html_str = FormBuilder.form(theme: :bootstrap_4_vertical, action: "/products", method: :post, form_html: {style: "margin-top: 20px;", "data-foo" => "bar"}) do |f|
  f << f.field(name: "name", type: :text, label: "Name")
  f << f.field(name: "sku", type: :text, label: "SKU")
  f << "<strong>Hello World</strong>"

OR you can use the lower level String.build instead:

form_html_str = String.build do |str|
  str << FormBuilder.form(theme: :bootstrap_4_vertical, action: "/products", method: :post, form_html: {style: "margin-top: 20px;", "data-foo" => "bar"}) do |f|
    str << f.field(name: "name", type: :text, label: "Name")
    str << f.field(name: "sku", type: :text, label: "SKU")
    str << "<strong>Hello World</strong>"

FormBuilder without a Form

- f = FormBuilder::Builder.new(theme: :bootstrap_4_vertical)

== f.field name: "name", type: :text, label: "Name"
== f.field name: "sku", type: :text, label: "SKU"

Error Handling

The form builder is capable of handling error messages too. If the :errors argument is provided it will generate the appropriate error help text element(s) next to the field.

== FormBuilder.form(theme: :bootstrap_4_vertical) do |f|
  == f.field name: "name", type: :text, label: "Name", errors: "cannot be blank"
  == f.field name: "sku", type: :text, label: "SKU", errors: ["must be unique", "incorrect SKU format")

Custom Themes

FormBuilder allows you to create custom themes very easily.

Example Usage:

FormBuilder.form(theme: :custom)

Example Theme Class:

# config/initializers/form_builder.cr

module FormBuilder
  module Themes
    class Custom < BaseTheme

      ### (Optional) If your theme name doesnt perfectly match the `.underscore` of the theme class name
      def self.theme_name

      ### (Optional) If your theme requires additional variables similar to `Bootstrap3Horizontal.new(columns: ["col-sm-3", "col-sm-9"])`
      def initialize
        ### For an example see `src/form_builders/themes/bootstrap_3_horizontal.cr`

      def wrap_field(field_type : String, html_field : String, html_label : String?, html_help_text : String?, html_errors : Array(String)?, wrapper_html_attributes : StringHash)
        String.build do |s|
          wrapper_html_attributes["class"] = "form-group #{wrapper_html_attributes["class"]?}".strip

          ### `FormBuilder.build_html_attr_string` is the one and only helper method for Themes
          ### It converts any Hash to an HTML Attributes String
          ### Example: {"class" => "foo", "data-role" => "ninja"} converts to "class=\"foo\" data-role=\"ninja\""
          attr_str = FormBuilder.build_html_attr_string(wrapper_html_attributes)

          s << "#{attr_str.empty? ? "<div>" : %(<div #{attr_str}>)}"

          if {"checkbox", "radio"}.includes?(field_type) && html_label && (i = html_label.index(">"))
            s << html_label.insert(i+1, "#{html_field} ")
            s << html_label
            s << html_field

          s << html_help_text

          if html_errors
            s << html_errors.join

          s << "</div>"

      def input_html_attributes(html_attrs : Hash(String, String), field_type : String, has_errors? : Bool)
        html_attrs["class"] = "form-field other-class #{html_attrs["class"]?}".strip
        html_attrs["style"] = "color: blue; #{html_attrs["style"]?}".strip

        unless html_attrs.has_key?("data-foo")
          html_attrs["data-foo"] = "bar #{html_attrs["class"]?}"


      def label_html_attributes(html_attrs : Hash(String, String), field_type : String, has_errors? : Bool)
        html_attrs["class"] = "form-label other-class #{html_attrs["class"]?}".strip
        html_attrs["style"] = "color: red; #{html_attrs["style"]?}".strip

      def form_html_attributes(html_attrs : Hash(String, String))
        html_attrs["class"] = "form-inline #{html_attrs["class"]}"

      def build_html_help_text(help_text : String, html_attrs : StringHash)
        html_attrs["class"] = "help-text #{html_attrs["class"]?}".strip

        String.build do |s|
          s << (html_attrs.empty? ? "<div>" : %(<div #{build_html_attr_string(html_attrs)}>)
          s << help_text
          s << "</div>"

      def build_html_error(error : String, html_attrs : StringHash)
        html_attrs["class"] = "help-text error #{html_attrs["class"]?}".strip
        html_attrs["style"] = "color: red; #{html_attrs["style"]?}".strip

        String.build do |s|
          s << (html_attrs.empty? ? "<div>" : %(<div #{build_html_attr_string(html_attrs)}>)
          s << error
          s << "</div>"


Now you can use the theme just like any other built-in theme.

FormBuilder.form(theme: :custom)


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Ruby Alternative

This library has been ported to the Ruby language as SexyForm.rb

The pattern/implementation of this form builder library turned out so beautifully that I felt the desire to have the same syntax available in the Ruby language. Many Crystal developers also write Ruby and vice versa so this only made sense. What was awesome is that, the Crystal and Ruby syntax is so similar that converting Crystal code to Ruby was straight forward and quite simple.


Created & Maintained by Weston Ganger - @westonganger

For any consulting or contract work please contact me via my company website: Solid Foundation Web Development

Solid Foundation Web Development Logo

Project Inspired By: