I want to generate named functions with fn and return them from the macro, I tried the following example:
(defmacro getfn
[namestr children]
`(fn fn-name# []
(println "Recursing" ~namestr)
(doall (map (fn [child#] (child#)) ~children))))
(def foo (getfn "foo" []))
(def bar (getfn "bar" [foo]))
(defn -main [& args]
(bar))
The resulting output is usually as expected:
Recursing bar
Recursing foo
However, when I run this compiled ahead-of-time (AOT) I get:
Recursing bar
Recursing bar
...
Recursing bar
Recursing bar
Exception in thread "main" java.lang.StackOverflowError
I find it pretty strange that bar keeps calling itself instead of foo, the only sensible reason for this is for the generated symbol fn-name# to leak outside its scope. Is this a bug in Clojure or intended behaviour?
Update: For clarity should mention that removing the fn-name# symbol and making the function anonymous fixes this problem. However, in my actual code I need to call it recursively sometimes, so naming it is necessary.
One solution I have for this problem is to use gensym to get a new symbol for each version of the macro, this would work by modifying the getfn as follows:
(defmacro getfn
[namestr children]
`(let [fn-name# (gensym)]
(fn fn-name# []
(println "Recursing" ~namestr)
(doall (map (fn [child#] (child#)) ~children)))))
This feels a bit unnecessary since by definition the fn name should be relevant only inside its own scope.
Update: Just tested with alpha releases and it seems Clojure 1.7.0-alpha3 and later work without this hack, Clojure 1.7.0-alpha2 and earlier are broken. Using this workaround is probably ok until stable version of 1.7.0 is released, unless someone can think of something better.
Related
This is probably a newbie question.
I'm trying to bind a keyboard macro to a key and have it available each time I load a file. I'm using lispbox.
This is the code I'm trying (the macro used here is just a placeholder):
(fset 'macro1
(lambda (&optional arg)
"Keyboard macro."
(interactive "p")
(kmacro-exec-ring-item (quote ("uuu" 0 "%d")) arg)))
(global-set-key "[f5]" 'macro1)
But when evaluating, fset and global-set-key are undefined. I think I managed to avoid using fset by doing:
(setf (symbol-function 'macro1)...
But I can´t work my way around global-set-key. Calling the variable global-map shows the same message. I'm guessing, after searching a lot, that this isn't meant to work en CL but in Elisp. How should I proceed?
This question is somewhat related to an earlier one on programmatically generating symbol macros. I'm using that function in a convenience macro that throws undefined variable warnings. This macro and function:
(defmacro define-data (d body &optional doc)
(if (and doc (not (stringp doc))) (error "Documentation is not a string"))
`(let* ((d-str (string ',d))
(old-package *package*)
(*package* (if (find-package d-str) ;exists?
(find-package d-str) ;yes, return it
(make-package d-str)))) ;no, make it
;; Should we have an eval-when (:compile-toplevel) here?
(defparameter ,d ,body ,doc)
(export ',d old-package)
(define-column-names ,d)))
(defun define-column-names (d)
(maphash #'(lambda (key index)
(eval `(cl:define-symbol-macro ,key (cl:aref (columns ,d) ,index))))
(ordered-keys-table (slot-value d 'ordered-keys))))
are intended to be like defparameter, but additionally set up a few niceties for the user by defining:
a package with the name of d
a parameter in the current package with the data that will be sucked in by body
symbol-macros in package d for access to the individual data vectors
If I use defparameter from the REPL, and then call define-column-names, all is well. However when using the macro I get:
; in: DEFINE-COLUMN-NAMES FOO
; (DEFINE-COLUMN-NAMES CL-USER::FOO)
;
; caught WARNING:
; undefined variable: CL-USER::FOO
I suspect that this is because the compiler has no way of knowing that FOO will actually be defined when define-symbol-macro is called. Everything works fine, but I don't want the warning to frighten users, so am thinking of suppressing it. I hate suppressing warnings though, so thought I'd come here for a second opinion.
EDIT: I've marked an answer correct because it does correctly answer the question as asked. For an answer to the problem see my comments.
My answer to the 'when to muffle warnings' question in the title is: if it's your own code then never, under any circumstances. If it is someone else's code, then rewrite it not to warn unless you can't.
As to solving the problem I haven't thought about this hard enough, but the problem is that you definitely want the defparameter to be at top-level so the compiler can see it, and it can't really be if it's inside a let. But you can raise it to toplevel trivially since it depends on nothing inside the let.
I am then pretty certain that you want the rest of the macro to happen at compile time, because you definitely want the symbol-macros available at compile-time. So an attempt at the first macro would be (note I've fixed the handling of the docstring: (defparameter foo 1 nil) is bad):
(defmacro define-data (d body &optional doc)
(when (and doc (not (stringp doc)))
(error "Documentation is not a string"))
`(progn
(defparameter ,d ,body ,#(if doc (list doc) '()))
(eval-when (:compile-toplevel :load-toplevel :execute)
(let* ((d-str (string ',d))
(old-package *package*)
(*package* (if (find-package d-str) ;exists?
(find-package d-str) ;yes, return it
(make-package d-str)))) ;no, make it
(export ',d old-package)
(define-column-names ,d)))))
As a side note: although I think the fact that programmatically defining symbol macros is hard because CL left that out for some reason, I think I'd personally use some other approach rather than this, because eval is just so horrid. That's just me however: if you want to do this you do need eval I think (it is very rare that this is true!).
I am not sure exactly how define-columns-names works so I replaced it with a stub function that returns d.
Note also that you can use check-type and should try not injecting symbols in generated code, this introduces potential variable capture that can be avoided with gensym.
As far as I know you cannot use eval-when as suggested by your comment (see Issue EVAL-WHEN-NON-TOP-LEVEL Writeup for details).
But I have no warning if I declare the symbol as being special around the call.
(defmacro define-data (d body &optional doc)
(check-type doc (or null string))
(check-type d symbol)
(let ((d-str (string d)))
(alexandria:with-gensyms (old-package)
`(let* ((,old-package *package*)
(*package* (if (find-package ,d-str) ;exists?
(find-package ,d-str) ;yes, return it
(make-package ,d-str)))) ;no, make it
(defparameter ,d ,body ,doc)
(export ',d ,old-package)
(locally (declare (special ,d))
(define-column-names ,d))))))
It is also a bit strange that you expand into a call to define-column-names, which in turns evaluated a form built at runtime. I think it might be possible to do all you want during macroexpansion time, but as said earlier what you are trying to do is a bit unclear to me. What I have in mind is to replace define-column-names by:
,#(expand-column-names-macros d)
... where expand-column-names-macros builds a list of define-symbol-macro forms.
I have a use case where I want to update one of my map type variables inside a method call. To demonstrate here is a code snippet,
(defn func [mymap]
(conj mymap [1 2])) ;update mymap variable here such that changes are persistent after the method returns
(let [mymap {}
_ (func mymap)] (println mymap))
which outputs {} because I think a new map is created with the conj function. How do I update the mymap variable in func such that the output of the above program will be {1 2}?
If it is not possible in Clojure, how are such use cases handled in general?
Many choices. Simplest is to rebind the mymap variable. Consider:
(ns tst.demo.core
(:use tupelo.core tupelo.test))
(defn doit [mymap]
(into mymap {:b 42}))
(dotest
(let [m {:a 1}
m2 (doit m)]
(spyx m2)))
m2 => {:a 1, :b 42}
and we get what we expect.
Update the code to reuse the name m:
(dotest
(let [m {:a 1}
m (doit m)]
(spyx m)))
m => {:a 1, :b 42}
Here the 2nd usage of m creates a separate variable that shadows the first m. This works great and people do it accidentally all the time without even realizing it.
If you want to copy the behavior of Java, you need a Clojure atom to create a mutable storage location.
(dotest
(let [m-atom (atom {:a 1})]
(swap! m-atom doit)
(spyx #m-atom)))
(deref m-atom) => {:a 1, :b 42}
Here swap! applies the function doit to the contents
of m-atom, the puts the results as the new contents.
We need the #m-atom or (deref m-atom) to pull out the contents of the atom for printing.
The above convenience functions can be found here. I also have some great documentation references in this template project. Be especially sure to study the Clojure Cheatsheet daily.
Clojure uses immutable data types by default. This means, you cannot
mutate the data in place like you are used to from many other
programming languages.
The functional approach here is to use the result from the conj (the
last statement inside a defn is it's return value).
(let [mymap {}
result (func mymap)]
(println result))
The longer you can keep up with pure functions on immutable data the
easier your life will be; reasoning about your programs and testing
them becomes a lot easier.
There is of course the option to use mutable data classes from Java, but
don't use them unless you really have to.
And since nearly all programs need some state, there are also atom:s
I only mention this here, because short of def everywhere, atom
everywhere are the next best "trap" beginners run into.
I've just been reading up on the sharpsign colon reader macro and it sounded like it had a very similar effect to gensym
Sharpsign Colon: "introduces an uninterned symbol"
Gensym: "Creates and returns a fresh, uninterned symbol"
So a simple test
CL-USER> #:dave
; Evaluation aborted on #<UNBOUND-VARIABLE DAVE {1002FF77D3}>.
CL-USER> (defparameter #:dave 1)
#:DAVE
CL-USER> #:dave
; Evaluation aborted on #<UNBOUND-VARIABLE DAVE {100324B493}>.
Cool so that fails as it should.
Now for the macro test
(defmacro test (x)
(let ((blah '#:jim))
`(let ((,blah ,x))
(print ,blah))))
CL-USER> (test 10)
10
10
CL-USER>
Sweet so it can be used like in a gensym kind of way.
To me this looks cleaner than gensym with an apparently identical result. I'm sure I'm missing a vital detail so my question is, What it it?
Every time the macro is expanded, it will use the same symbol.
(defmacro foo () `(quote #:x))
(defmacro bar () `(quote ,(gensym)))
(eq (foo) (foo)) => t
(eq (bar) (bar)) => nil
Gensym will create a new symbol every time it is evaluated, but sharp colon will only create a new symbol when it is read.
While using sharp colon is unlikely to cause problems, there are a couple rare cases where using it would lead to nearly impossible to find bugs. It is better to be safe to begin with by always using gensym.
If you want to use something like sharp colon, you should look at the defmacro! macro from Let Over Lambda.
GENSYM is like MAKE-SYMBOL. The difference is that GENSYM supports fancy naming by counting up -> thus symbols kind of have unique names, which makes debugging a bit easier when having gensyms for example in macro expansions.
#:foo is a notation for the reader.
So you have a function which creates these and a literal notation. Note that, when *print-circle* is true, some kind of identity maybe preserved in s-expressions: #(#1=#:FOO #1#).
Generally this is similar to (a . b) and (cons 'a 'b), #(a b) and (vector 'a 'b)... One is literal data and the other one is a form which will create ('cons') fresh objects.
If you look at your macro, the main problem is that nested usage of it could cause problems. Both lexically or dynamically.
lexically it could be the same variable, which is rebound.
dynamically, if it is a special variable it could also be rebound
Using a generated symbol at macro expansion time would make sure that different and expanded code would not share bindings.
I am learning Clojure by solving problems, I'm stuck with one of them, basically I have to find the top five strings in a log file.
Here is what I've got so far:
(ns topfive
(:import (java.io BufferedReader FileReader)))
(defn extract-query [line]
(.substring line (+ (.lastIndexOf line "=") 1) (.lastIndexOf line "]")))
(defn process-file [file-name, queries]
(with-open [rdr (BufferedReader. (FileReader. file-name))]
(doseq [line (line-seq rdr)]
(assoc queries (extract-query line) (inc (get queries (extract-query line) 0))))))
(process-file "in" {})
My problem is that queries does not contain anything, I've already checked that extract-queries returns the string I want, I thought that this might have something to do with the language itself, I've read that Clojure has immutability at language level, but this still does not seem a good point to me.
Could you suggest something about what I am doing wrong?
Clojure does have immutability at a low level, and hash-maps are immutable. So assoc doesn't mutate a map in-place, it creates a new map with an updated item in it, and returns the new map. You're calling assoc over and over, but discarding the results.
One fix is to use reduce instead of doseq. doseq iterates over a seq and does something to each item, but doesn't accumulate any results. So it should be used mostly for things that have side effects, e.g. printing to screen or file. reduce similarly iterates over a seq, but it does accumulate results.
(defn process-file [file-name, queries]
(with-open [rdr (BufferedReader. (FileReader. file-name))]
(reduce (fn [queries, line]
(assoc queries (extract-query line) (inc (get queries (extract-query line) 0))))
queries
(line-seq rdr))))
You could do a few things to simplify this a bit further. There's no need for a queries parameter to process-file, since it's always going to be an empty map to begin with. Your assoc line can be written more concisely using update-in and fnil; this also lets us avoid calling extract-query twice per line. You can replace all the calls to the Java Reader classes with the Clojure wrapper reader in clojure.java.io. You can replace your calls to substring with a regular expression; regex is more concise, but for large inputs your version might perform faster. You could also replace the anonymous function in my example with a sugary reader macro version using #(), though it's starting to look a bit noisy at this point, so I'd probably use let to make it read a bit better.
(ns topfive
(:require [clojure.java [io :as io]]))
(defn extract-query [line]
(nth (re-find #"query=([^]]+)" line) 1))
(defn process-file [file-name]
(with-open [rdr (io/reader file-name)]
(reduce #(let [search-term (extract-query %2)]
(update-in %1 [search-term] (fnil inc 0)))
{}
(line-seq rdr))))
in addition to Brians excellent answer: The threading macro may improve readability:
(ns stackoverflow
(:use [clojure.string :only [split]]
[clojure.java.io :only [reader]]))
(->> (reader "input.txt")
(line-seq)
(map #(last (split % #"=")))
(frequencies))