I have a :get route with easy-routes that, when hit, runs one function (quick-test). The function quick-test returns two values. Both are strings.
(easy-routes:defroute test-file ("/test-file" :method :get) (&get filename)
(quick-test filename))
However, when the form is sent to "/test-file", only one line is visible in the dom.
I then tried using multiple value bind. But the result is the same.
(easy-routes:defroute test-file ("/test-file" :method :get) (&get filename)
(multiple-value-bind (a b) (quick-test filename)
(values (format nil "~a" a)
(format nil "~a" b))))
My goal is to be able to run quick test and see all returned values on the dom.
Is there a way to do that?
For completeness, here is the form:
<form id="form" action="/test-file" method="get">
<input type="text" name="filename" placeholder="type the filename"/>
<input type="submit" value="Send"/>
</form>
Values can return more values, but if your function expects some expression, it takes only the first returned value and discards the rest. For example, floor returns two values, but + takes only the first:
> (floor 5 2)
2
1
> (+ 2 (floor 5 2))
4
This question can also help you: values function in Common Lisp
Defroute probably does the same and takes only the first value.
You can fix your route for example like this:
(easy-routes:defroute test-file ("/test-file" :method :get) (&get filename)
(multiple-value-bind (a b) (quick-test filename)
(format nil "~a, ~a" a b)))
Related
The usual way to generate HTML using CL-WHO is by using the macros with-html-output and with-html-output-to-string. This uses special syntax. For example:
(let ((id "greeting")
(message "Hello!"))
(cl-who:with-html-output-to-string (*standard-output*)
((:p :id id) message)))
Is it possible to write the data ((:p :id id) message) as a list instead of using the macro syntax shown above? For example, I would like to define the HTML as a list like this:
(let* ((id "greeting")
(message "Hello!")
(the-html `((:p :id ,id) ,message)))
;; What should I do here to generate HTML using CL-WHO?
)
Can CL-WHO take a normal Lisp list and produce HTML from the list?
You want to insert code into an expression.
Actually you would need eval:
(let* ((id "greeting")
(message "Hello!")
(the-html `((:p :id ,id) ,message)))
(eval `(cl-who:with-html-output-to-string (*standard-output*)
,the-html)))
But this is not good to use eval.
But a macro contains an implicite eval.
I would define a macro for this and call the macro:
(defun html (&body body)
`(cl-who:with-html-output-to-string (*standard-output*)
,#body))
;; but still one doesn't get rid of the `eval` ...
;; one has to call:
(let* ((id "greeting")
(message "Hello!")
(the-html `((:p :id ,id) ,message)))
(eval `(html ,the-html)))
I am working on a function that transforms an alist into a query parameters. So far it looks like this.
(defun encode-options (opts)
"Turns an alist into url query parameters."
(format nil "~{~{~A=~A~}~^&~}" opts))
This works perfectly for alists like ((a b) (c d)) (Resulting in "A=B&C=D"), but fails for dotted alists like ((a . b) (c . d)). (Resulting in The value B is not of type LIST.)
My question is: Is it possible to format the dotted alist to give me the expected results and how?
Is it possible to format the dotted alist?
No, format iterates over proper lists.
There are many possible ways to implement what you want. Here I present two of them.
Keep control string, change data
(defun ensure-proper-list (value)
(typecase value
(null nil)
(cons (cons (car value)
(ensure-proper-list (cdr value))))
(t (list value))))
Now, you transform the option argument so that all elements are proper lists:
(defun encode-options (options)
"Turns an alist into url query parameters."
(format nil
"~{~{~A=~A~}~^&~}"
(mapcar #'ensure-proper-list options)))
Keep data, change control string
(defun print-alist (stream data &optional colonp atsignp)
(declare (ignore colonp atsignp))
(destructuring-bind (head . tail) data
(format stream "~A=~A" head (if (consp tail) (first tail) tail))))
With this new format control, print the list as given:
(defun encode-options (options)
"Turns an alist into url query parameters."
(format nil
"~{~/lib:print-alist/~^&~}"
options))
Note that I added a package prefix lib because without a package, print-alist would be looked up in the user package (a.k.a. COMMON-LISP-USER), which in my opinion is rarely what you want. From 22.3.5.4 Tilde Slash: Call Function:
The function corresponding to a ~/name/ directive is obtained by
looking up the symbol that has the indicated name in the indicated
package. If name does not contain a ":" or "::", then the whole name
string is looked up in the COMMON-LISP-USER package.
That's why I would recommend to always mention the package with ~/ directives.
I am writing a common lisp program and I have a variable that can contain either a string or a function. I want to call the function if it is one and return that as well as the string. How do I test if a variable is a function?
Code so far:
(defun string-or-function (var)
(if (typep var 'simple-array)
var
(if "Function equivalent of typep goes here."
(setf temp (fn-that-does-something))
(string-or-function temp)
Edit: Code that works:
(defun string-or-function (var)
(let ((s-or-f (type-of var)))
(if (equal s-or-f 'function)
(print "function")
(if (equal (car s-or-f) 'simple-array)
(print "string")))))
Is there a better way to do it?
Common Lisp has a predicative type system. The notion that a value has a "principal" type doesn't make as much sense in Lisp. The type-of function is actually fairly infrequently used, as it makes less sense to ask "What is the type of X" and more sense to ask "Is X of type Y". This can be done with typep, or in your case more concisely with typecase, which is just a case statement for types.
(defun string-or-function (var)
(typecase var
(string (format t "string"))
(function (format t "function"))
(t (format t "something else"))))
I want to call the function if it is one and return that as well as the string.
I think you mean something like this:
(defun evaluate (arg)
"Returns something from evaluating ARG in some manner. If ARG is a string,
return it. If ARG is a function, call it with no arguments and return its
return value(s)."
(ctypecase arg
(string arg)
(function (funcall arg))))
If you need extensibility:
(defgeneric evaluate (arg)
(:documentation "Returns something from evaluating ARG in some manner."))
(defmethod evaluate ((arg string))
arg)
(defmethod evaluate ((arg function))
(funcall arg))
Here are some other ways:
(defun string-or-function-p (x)
(typep x '(or string function)))
...but you can probably also use check-type, which is not a predicate but a check which signals a restartable condition in case the value does not satisfy the type specification:
(check-type place (or string function))
If you happen to use this type a lot, define a custom type:
(deftype string-or-fun () '(or string function))
Of course, you can also use generic functions depending on your needs (silly example):
(defgeneric execute (object)
(:method ((s string)) (eval (read-from-string s)))
(:method ((f function)) (funcall f)))
But note that generic function dispatch on classes, not types, which are different things.
(eq (type-of var) 'function)
However, remember that Common Lisp keeps variables and function names in different namespaces, so (var 1 2 3) and (cons var 1) are looking in two different places. You probably cannot call var like (var), but will rather need to use (funcall var), depending on which namespace this is in.
Basically, you probably shouldn't be stuffing a function-or-maybe-a-string into one variable.
I want to retrieve the string, generated by write for further processing without doing any actual output, but write seems always to also output into the REPL
CL-USER>(let ((err-string (write (make-instance 'error) :stream nil)))
(do-awesome-stuff-with-string err-string))
<ERROR> ;;this is the printing I want to get rid of
"awesome-result"
Why does write still outputs into the REPL, and how do I get rid of that?
You can use with-output-to-string for this. Here's an example:
(flet ((do-awesome-stuff-with-string (string)
(concatenate 'string string " is awesome!")))
(let ((err-string (with-output-to-string (s)
(write (make-instance 'error) :stream s))))
(do-awesome-stuff-with-string err-string)))
;; => "#<ERROR {25813951}> is awesome!"
Here's here's the HyperSpec entry on with-output-to-string.
The reason (write (make-instance 'error) :stream nil) doesn't work is that the :stream argument to write is a stream designator and in that context nil is shorthand for *standard-output*. (The fact that format instead takes nil to mean that it should return a string is a common point of confusion).
Keep in mind that portably errors are made with MAKE-CONDITION. The standard does not say that errors are CLOS classes, so MAKE-INSTANCE might not work in some implementations.
There are two simple ways to get a string:
a) a textual description:
CL-USER 15 > (princ-to-string (make-condition 'error))
"The condition #<ERROR 4020311270> occurred"
b) the error object printed:
CL-USER 16 > (prin1-to-string (make-condition 'error))
"#<ERROR 402031158B>"
I am curious how
(write
(make-instance 'simple-error
:format-control "A:~a ~% B:~a~%"
:format-arguments `("A" "B"))
:stream nil)
works, as I tried to implement it myself to gain experience in basic lisp funcionality but soon had to realize, that I am not able to. As the intuitive way of implementation:
(defmethod my-write ((simple-error err))
(FORMAT nil (if (simple-condition-format-control err)
(simple-condition-format-control err)
"")
(simple-condition-format-arguments err)))
obviously cannot work, as (simple-condition-format-arguments err) returns the list of arguments and therefore, in the example above, "B:~a" does not have a corresponding parameter to print.
So how would I actually implement this method?
You can use apply for this. It takes the function passed as its first argument and applies it to arguments constructed from its other arguments. For example, (apply #'f 1 2) calls (f 1 2), (apply #'f 1 '(2 3)) calls (f 1 2 3) and so on. It's perfectly suited for this situation.
SBCL has a function almost identical to yours:
(defun simple-condition-printer (condition stream)
(let ((control (simple-condition-format-control condition)))
(if control
(apply #'format stream
control
(simple-condition-format-arguments condition))
(error "No format-control for ~S" condition))))
As mentioned by Samuel, you need to use APPLY.
Also note that NIL for the stream in WRITE does something else than in FORMAT. With FORMAT the stream argument NIL causes the output to be returned as a string. With man other output functions, like WRITE, it means standard output.