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trivial-string-template

This is a simple templating engine. It will be useful if you want to use user’s input as the templates and dont want to give a user the full power of Lisp’s format.

There is a function which replaces placeholders in the template:

POFTHEDAY> (trivial-string-template:substitute "$who likes $what"
                                               :who "Bob"
                                               :what "Common Lisp")
"Bob likes Common Lisp"

;; Also, you can use curly braces around the placeholder:

POFTHEDAY> (trivial-string-template:substitute "${who} likes ${what}"
                                               :who "Bob"
                                               :what "Common Lisp")
"Bob likes Common Lisp"

;; If some variable wasn't given, it will raise an error:

POFTHEDAY> (trivial-string-template:substitute "${who} likes ${what}"
                                               :who "Bob")
; Debugger entered on #<SIMPLE-ERROR "Missing variable ~A~A information." {100687A6C3}>

;; But there is a safe version of the function which just ignores:

POFTHEDAY> (trivial-string-template:safe-substitute "${who} likes ${what}"
                                                    :who "Bob")
"Bob likes $what"

Also, you can define a template object. It will be funcallable and can be used in two ways.

The first way - to create a template object and funcall it:

POFTHEDAY> (trivial-string-template:template "${who} likes ${what}")
#<TRIVIAL-STRING-TEMPLATE:TEMPLATE {10076CA14B}>

POFTHEDAY> (funcall *
                    :who "Bob"
                    :what "Common Lisp")
"Bob likes Common Lisp"

POFTHEDAY> (funcall **
                    :who "Mary"
                    :what "Common Lisp")
"Mary likes Common Lisp"

The second way is to define a function:

POFTHEDAY> (trivial-string-template:define-template who-♥️-what ()
                                                    "${who} likes ${what}")
#<TRIVIAL-STRING-TEMPLATE:TEMPLATE {100635045B}>

POFTHEDAY> (describe 'who-♥️-what)

POFTHEDAY::WHO-♥️-WHAT
  [symbol]

WHO-♥️-WHAT names a compiled function:
  Lambda-list: (&KEY (WHO NIL WHO-SUPPLIED-P)
                (WHAT NIL WHAT-SUPPLIED-P))
  Derived type: (FUNCTION (&KEY (:WHO T) (:WHAT T))
                 (VALUES SIMPLE-STRING &OPTIONAL))
  Source form:
    (LAMBDA (&KEY (WHO NIL WHO-SUPPLIED-P) (WHAT NIL WHAT-SUPPLIED-P))
      (PROGN
       (UNLESS WHO-SUPPLIED-P
         (ERROR
          #1="The variable ~A~A is not supplied, which must be supplied in non-safe mode."
          #\$ "who"))
       (UNLESS WHAT-SUPPLIED-P (ERROR #1# #\$ "what"))
       (FORMAT NIL "~A likes ~A" WHO WHAT)))

POFTHEDAY> (who-♥️-what :who "Bob" :what "LISP")
"Bob likes LISP"

What is interesting is that the template object uses this meta class: closer-mop:funcallable-standard-class. It makes possible to create instances which can be called the same way as the functions.

Here is an example of how to make a class for which instances will be funcallable:

POFTHEDAY> (defclass foo ()
             ()
             (:metaclass closer-mop:funcallable-standard-class))
#<SB-MOP:FUNCALLABLE-STANDARD-CLASS POFTHEDAY::FOO>

POFTHEDAY> (defmethod initialize-instance :after ((this foo) &key)
             (closer-mop:set-funcallable-instance-function
              this
              (lambda ()
                (format t "Hello Lisp World!~%"))))
#<STANDARD-METHOD COMMON-LISP:INITIALIZE-INSTANCE :AFTER (FOO) {10020B09B3}>

POFTHEDAY> (make-instance 'foo)
#<FOO {10020B585B}>

POFTHEDAY> (funcall *)
Hello Lisp World!
NIL

Such objects can be used instead of closure in cases where you will need to redefine some of the instance slots after the object was created.

Ok, now it is time to say goodbye. See you tomorrow in the next #poftheday post!