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Formspec Library (fslib)

A tiny Minetest library mod to facilitate managing formspecs.

Written by Lars Müller alias appgurueu and licensed under the terms of the MIT license.




Builds a formspec string out of a table of S-expressions; purely a string building helper entirely unaware of formspec semantics: Everything must properly use the table formats no matter the context.

Subtables of formspec tables take the form {"element_name", ...}, where ... may consist of:

  • Strings to be escaped;
  • Numbers to be formatted using %d (integers) or %f (floats);
  • Booleans to be formatted as true or false;
  • A subtable:
    • An options table {[option] = value}
    • A sublist to which the same formatting rules apply but which will use , instead of ; as delimiter (e.g. coordinates);
    • A hypertext table created by through the use of fslib.hypertext_root{fslib.hypertext_tags.tag{attr = value, child_1, child_2, "text", ...}}
      • Tip: Localize local tags = fslib.hypertext_tags

formspec_name = fslib.show_formspec(player, formspec, handler)

  • player: PlayerRef (ObjectRef)
  • formspec: Formspec in string or table format (see fslib.build_formspec)
  • handler: function(fields) return next_formspec which is called as the form is submitted
    • fields: Submitted formspec fields as provided by the engine
    • next_formspec: If returned, the formspec will be reshown by sending the next_formspec; player & handler stay the same
    • Tip: Use upvalues of handler - including the player - as context!
  • Returns an ID formspec_name which can be used to reshow the formspec
    • Tip: If you don't need to reshow the formspec with the same name or can reshow it by returning a new formspec inside the handler you should not use this.

fslib.reshow_formspec(player, formspec_name, formspec)

Reshows ("updates") the formspec reusing the name. formspec may be a formspec_table.


Closes whichever formspec was last shown to player using fslib.

Tip: If possible use exit elements in the formspec instead for a better user experience and to work around a bug.


local delete_confirmation_fs = fslib.build_formspec{
    {"size", {6, 1, false}},
    {"real_coordinates", true},
    {"label", {0.25, 0.5}; "Irreversably delete paintable?"},
    {"image_button_exit", {4.75, 0.25}; {0.5, 0.5}; "epidermis_check.png"; "confirm"; ""},
    {"tooltip", "confirm", "Confirm"},
    {"image_button_exit", {5.25, 0.25}; {0.5, 0.5}; "epidermis_cross.png"; "close"; ""},
    {"tooltip", "close", "Close"},
function entity_def:_show_delete_formspec(player)
    fslib.show_formspec(player, delete_confirmation_fs, function(fields)
        if fields.confirm then

(from epidermis, slightly modified)


  • The (static) formspec is built at load time to improve performance
  • The formspec field handler obtains self - in this case an entity - as context



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