I have a Clojure function with a post-condition :
{:post [(s/valid? ::MyQuery %)]}
But I'm getting this error :
Exception in thread "main" java.lang.RuntimeException: Unable to resolve symbol: % in this context, compiling:(polvo/core.clj:45:11)is.
It seems to not know what the % is.
But in every example I can find of Clojure :post that's what it uses to refer to the return value.
Any ideas what's going on?
Most likely explanation: your pre/post map is not in the right position.
Related
I find a macro .?. on the website https://clojure.github.io/clojure-contrib/core-api.html. But I can't use it. I tried the example,
(.?. "foo" .toUpperCase (.substring 1))
the clojure repl returns an error:
user=> (.?. "foo" .toUpperCase (.substring 1))
Syntax error compiling at (REPL:1:1).
Unable to resolve symbol: .toUpperCase in this context
I notice the namespace of .?. is deprecated. Can't we use this macro anymore?
It has been moved to core.incubator, like #phipsgabler said.
You should use some-> from clojure.core.
(some-> "foo" (.toUpperCase) (.substring 1))
It has been moved to a separate module core.incubator.
I'm fetching thousands of entities from an API one at a time using http requests. As next step in the pipeline I want to shovel all of them into a database.
(->> ids
(pmap fetch-entity)
(pmap store-entity)
(doall))
fetch-entity expects a String id and tries to retrieve an entity using an http request and either returns a Map or throws an exception (e.g. because of a timeout).
store-entity expects a Map and tries to store it in a database. It possibly throws an exception (e.g. if the Map doesn't match the database schema or if it didn't receive a Map at all).
Inelegant Error Handling
My first "solution" was to write wrapper functions fetch-entity' and store-entity' to catch exceptions of their respective original functions.
fetch-entity' returns its input on failure, basically passing along a String id if the http request failed. This ensures that the whole pipeline keeps on trucking.
store-entity' checks the type of its argument. If the argument is a Map (fetch entity was successful and returned a Map) it attempts to store it in the database.
If the attempt of storing to the database throws an exception or if store-entity' got passed a String (id) instead of a Map it will conj to an external Vector of error_ids.
This way I can later use error_ids to figure out how often there was a failure and which ids were affected.
It doesn't feel like the above is a sensible way to achieve what I'm trying to do. For example the way I wrote store-entity' complects the function with the previous pipeline step (fetch-entity') because it behaves differently based on whether the previous pipeline step was successful or not.
Also having store-entity' be aware of an external Vector called error_ids does not feel right at all.
Is there an idiomatic way to handle these kinds of situations where you have multiple pipeline steps where some of them can throw exceptions (e.g. because they are I/O) where I can't easily use predicates to make sure the function will behave predictable and where I don't want to disturb the pipeline and only later check in which cases it went wrong?
It is possible to use a type of Try monad, for example from the cats library:
It represents a computation that may either result in an exception or return a successfully computed value. Is very similar to the Either monad, but is semantically different.It consists of two types: Success and Failure. The Success type is a simple wrapper, like Right of the Either monad. But the Failure type is slightly different from Left, because it always wraps an instance of Throwable (or any value in cljs since you can throw arbitrary values in the JavaScript host).(...)It is an analogue of the try-catch block: it replaces try-catch’s stack-based error handling with heap-based error handling. Instead of having an exception thrown and having to deal with it immediately in the same thread, it disconnects the error handling and recovery.
Heap-based error-handling is what you want.
Below I made an example of fetch-entity and store-entity. I made fetch-entity throw an ExceptionInfo on the first id (1) and store-entity throws a DivideByZeroException on the second id (0).
(ns your-project.core
(:require [cats.core :as cats]
[cats.monad.exception :as exc]))
(def ids [1 0 2]) ;; `fetch-entity` throws on 1, `store-entity` on 0, 2 works
(defn fetch-entity
"Throws an exception when the id is 1..."
[id]
(if (= id 1)
(throw (ex-info "id is 1, help!" {:id id}))
id))
(defn store-entity
"Unfortunately this function still needs to be aware that it receives a Try.
It throws a `DivideByZeroException` when the id is 0"
[id-try]
(if (exc/success? id-try) ; was the previous step a success?
(exc/try-on (/ 1 (exc/extract id-try))) ; if so: extract, apply fn, and rewrap
id-try)) ; else return original for later processing
(def results
(->> ids
(pmap #(exc/try-on (fetch-entity %)))
(pmap store-entity)))
Now you can filter results on successes or failures with respectively success? or failure? and retrieve the values via cats-extract
(def successful-results
(->> results
(filter exc/success?)
(mapv cats/extract)))
successful-results ;; => [1/2]
(def error-messages
(->> results
(filter exc/failure?)
(mapv cats/extract) ; gets exceptions without raising them
(mapv #(.getMessage %))))
error-messages ;; => ["id is 1, help!" "Divide by zero"]
Note that if you want to only loop over the errors or successful-results once you can use a transducer as follows:
(transduce (comp
(filter exc/success?)
(map cats/extract))
conj
results))
;; => [1/2]
My first thought is to combine fetch-entity and store-entity into a single operation:
(defn fetch-and-store [id]
(try
(store-entity (fetch-entity id))
(catch ... <log error msg> )))
(doall (pmap fetch-and-store ids))
Would something like this work?
I am trying to build server-side rendering for om.next (1.0.0-alpha47). At some point I have to create a reconciler from Clojure:
(om/reconciler {})
(om/reconciler
{:state (atom {})
:normalize true
:parser (om/parser {})})
However evaluating any of these in my REPL gives:
Unhandled java.lang.IllegalArgumentException Multiple methods in
multimethod 'print-method' match dispatch value: class
om.next.Reconciler -> interface clojure.lang.IDeref and interface
clojure.lang.IRecord, and neither is preferred
How do I fix that?
I came to this solution via trial and error:
(prefer-method print-method clojure.lang.IPersistentMap clojure.lang.IDeref)
This seems to solve the conflict, sorry that I can't explain any details.
I'm trying to implement an interface that has properties but can't quite seem to get it to work and I also have not found any relevant examples via Google (yet). I'm sure I'm doing something completely wrong here but have no idea how to fix it.
(System.Reflection.Assembly/LoadWithPartialName "System.Web")
; naive, just trying to figure out how to implement the IHttpHandler interface in Clojure
(defn foo-handler []
(reify System.Web.IHttpHandler
(IsReusable [] false)
(ProcessRequest [context] ())))
IsReusable is a property and I don't know how to tell reify that it is not a traditional function.
CompilerException clojure.lang.CljCompiler.Ast.ParseException: Must supply at least one argument for 'this' in: IsReusable
Okay, I supply 'this' for IsReusable
CompilerException clojure.lang.CljCompiler.Ast.ParseException: Can't define method not in interfaces: IsReusable
I've also tried proxy but I get similar results.
I've also tried naming IsReusable to get_IsReusable which doesn't actually make a difference and I get the same compiler errors as above.
I've also tried deftype but I get a completely different error:
(deftype foo-handler []
System.Web.IHttpHandler
(get_IsReusable [this] false)
(ProcessRequest [this context] ()))
Compiler error:
InvalidCastException Unable to cast object of type 'clojure.lang.Var' to type 'System.Type'. clojure.lang.Namespace.ReferenceClass
Update:
The code posted for deftype works, I cannot reproduce the error that I posted above. I have no idea now what I was doing wrong at the time.
This took me a few hours of research and trial and error but I finally have success!
user=> (def foo-handler
(reify System.Web.IHttpHandler
(get_IsReusable [this] false)
(ProcessRequest [this context] ())))
#'user/foo-handler
user=>
Success!
user=> (instance? System.Web.IHttpHandler foo-handler)
true
This way is better and works fine from an ASP.NET application:
(deftype foo-handler []
System.Web.IHttpHandler
(get_IsReusable [this] false)
(ProcessRequest [this context]
(.Write (.Response context) "Hello, From Clojure CLR!")))
I have a Clojure proxy statement that was getting large and messy, so I decided to try factoring the code of the beginDrag method redefinition out of the proxy statement, like this:
(defn enhanced-start-drag
""
[pie]
(let [pobj (. pie getPickedNode)
pobj-coll (seq (.. pie getInputManager
getKeyboardFocus getSelection))]
(println pobj)
(println pobj-coll)
(println "----------")
(proxy-super startDrag pie))) ; THIS IS LINE 94 (SEE ERROR MSG)
(defn custom-selection-event-handler [marqueeParent selectableParent]
(proxy [PSelectionEventHandler] [marqueeParent selectableParent]
(decorateSelectedNode [node]
(let [stroke-color (Color/red)]
(.setStrokePaint node stroke-color)))
(undecorateSelectedNode [node]
(let [stroke-color (Color/black)]
(.setStrokePaint node stroke-color)))
(startDrag [pie] ; pie is a PInputEvent
(enhanced-start-drag pie))
(endStandardSelection [pie] ; pie is a PInputEvent
(let [pobj (.getPickedNode pie)
slip (. pobj getAttribute "slip")
]
(swap! *last-slip-clicked*
(fn [x] slip))))))
I get the following compile error:
cd /Users/gw/tech/clojurestuff/cljprojects/infwb/src/infwb/
1 compiler notes:
Unknown location:
error: java.lang.Exception: Unable to resolve symbol: this in this context
core.clj:94:5:
error: java.lang.Exception: Unable to resolve symbol: this in this context
(core.clj:94)
Compilation failed.
As soon as I restore the body of enhanced-start-drag into the body of the proxy statement, everything works.
My question: Is there a way to move the messy details out to a separate function to improve the readability of my code?
Thanks for all your ideas and solutions.
UPDATE, 10/27/11: See the comments below. Arthur Ulfeldt was sharp in pointing out that the issue is captured references, and Dave Ray is also correct in saying that all you have to do is add this as a parameter to enhanced-start-drag and then proxy-super will work correctly. When I made the following two changes (without any changes to the body of enhanced-start-drag), my code was working again:
(defn enhanced-start-drag
""
[pie this]
and
(startDrag [pie] ; IN THE PROXY STMT IN custom-selection-event-handler
(enhanced-start-drag pie this))
BTW, my project uses Dave Ray's seesaw project to get a Java Swing UI. seesaw is awesome, as are its docstrings and sample code (which are much better than most commercial software). I highly recommend it! And thank you, Dave!
You have been bitten by symbol capture. In this case it is intentaional though you need to stay aware of it. From the doc for proxy-super
Use to call a superclass method in the body of a proxy method.
Note, expansion captures 'this`
proxy is creating a class that calls a function, when the call gets into enhanced-start-drag the value of this is not where proxy-super expects
you may needs to pass this as another argument into enhanced-start-drag and then call (. saved-this ...) instead of using proxy-super.