I ran into the following error which is quite surprising. I added a field to the AppSettings in a Yesod app (using the Yesod scaffholding) and to my surprise, everything compiled even though I didn't do anything else (I was expecting to have to add somewhere a default value to the construction of AppSettings, but not). I got a runtime error instead telling me that a field was missing.
It appears that the only construction to AppSetting uses the RecordWildCards extension and looks like AppSettings{..}. Not defining the new field didn't generate an error but a warning (I didn't see it, because I was running test in continuous mode using stack test --file-watch). How is that possible ?
I try to reproduce the problem in a simple file and I get an error not a warning. So why do I get a warning for Yesod ? Is it a compilation flag or something ?
Edit
This is not specific to Yesod. I've made the test again with a simple file and it generates a warning not an error.
According to changelog in GHC, "that not a bug, it's a feature": https://ghc.haskell.org/trac/ghc/ticket/5334
You can change this behavior by changing type of your fields to strict (prepend a ! to type name - like !Int) - however, then you lose laziness (more about effects of strict types: Advantages of strict fields in data types)
Of course, you can also make it an error by slamming in -Werror compilation option, but then you need to be very strict about your code (no unused imports, no unused variables, even when unpacking a record etc.), or get rid of -Wall and turn on only warnings you perceive as important.
Related
In a Haskell project, I am using a dependency which I know contains type error. But that's actually fine as I never call this code.
So I want to enable defer-type-errors but only for that dependent package.
Is there a way to scope that compiler instruction somewhere (stack ? cabal?)
If you really have to you can set ghc options per package in stack.yaml, namely:
ghc-options:
your_package_name: -fdefer-type-errors
I'm not sure whether it's compatible with ghcjs.
But please be sure to make the users of your package aware, maybe include a disclaimer in the document in big bold fonts.
For an Windows app, the option for the default language is found in Package.appxmanifest under the project. However, I do not know where to find the setting for a class library, because there is nothing like Package.appxmanifest under a class library project.
As a result, after I add Strings\zh-CN\Resources.resw and a few strings into a class library project, I get a few warnings, saying:
MakePRI : warning 0xdef00522: Resources found for language(s) 'zh-cn' but no resources found for default language(s): 'en-US'. Change the default language or qualify resources with the default language. http://go.microsoft.com/fwlink/?LinkId=231899
MakePRI : warning 0xdef01051: No default or neutral resource given for '...'. The application may throw an exception for certain user configurations when retrieving the resources.
No actual help from "fwlink" as usual.
Although nothing bad actually happens because the app gets zh-CN as its default language, it is still quite annoying and I would like to eliminate the warnings to turn on "treat warnings as errors".
Try to manually edit the <DefaultLanguage> in the csproj file. It's under the first <PropertyGroup> subtree.
Here is a snippet of the context where you should be able to find this tag:
<DefaultLanguage>zh-CN</DefaultLanguage>
<TargetPlatformIdentifier>UAP</TargetPlatformIdentifier>
<TargetPlatformVersion>10.0.10240.0</TargetPlatformVersion>
<TargetPlatformMinVersion>10.0.10240.0</TargetPlatformMinVersion>
<MinimumVisualStudioVersion>14</MinimumVisualStudioVersion>
I swear I saw a new feature in a recent set of GHC release notes - but now I can find no reference to it. Am I delusional, or does this feature actually exist?
It was to do with loading incomplete modules. As best as I can remember, it allows you to turn off compilation errors due to undefined variables. (Naturally, at run-time this causes an exception to be thrown if you try to actually use the undefined variables for anything.) Does that sound familiar? Or is my mind making this up?
You are looking for a compile time option, vs a language extension, of "defer errors to runtime". That is, compile with -fdefer-type-errors.
I have a project that is running fine on Android and WinPhone 8. When I attempt to run on iOS, I've getting the following error
Microsoft.Practices.ServiceLocation.ActivationException: Cannot
register: No public constructor found in x
where x is whatever SimpleIoc.Default.Register<T, TU>(); the flow hits first. I've moved the code around (as suggested elsewhere) to ensure all of the platform specific SimpleIoc calls are made in ViewModelLocator.
I've added public default ctors in the classes that are complaining about the error (I have though set the PreferredConstructor to the original, not the newly added public ctor).
I have a feeling that this error is a false positive (something else is failing, but pointing at that code).
Using Xam.iOS via a build server (the code is coming from VS2015). Xcode is running the 8.3 emulators (it may need updating to allow for 8.4 testing)
It could be that the Linker is optimising away the constructor, if it thinks it's not used. Try setting the Linker Options to "Don't Link" and see if it does it again, or even new-up an instance of the class elsewhere so that the Linker knows that the constructor is used. You don't necessarily want to leave it that way, but if it eliminates the error, you'll at least know the reason.
The [Preserve] attribute did the trick for me.
Decorate constructor with it and keep your linker settings.
This attribute is part of the Microsoft.WindowsAzure.MobileServices namespace.
I have an erlang program, compiled with rebar, after the new debian release, it won't compile anymore, complaining about this:
-import(erl_scan).
-import(erl_parse).
-import(io_lib).
saying:
bad import declaration
I don't know erlang, I am just trying to compile this thing.
Apparently something bad happened to -import recently http://erlang.org/pipermail/erlang-questions/2013-March/072932.html
Is there an easy way to fix this?
Well, -import(). is working but it does NOT do what you are expecting it to do. It does NOT "import" the module into your module, nor does it go out, find the module and get all the exported functions and allow you to use them without the module name. You use -import like this:
-import(lists, [map/2,foldl/3,foldr/3]).
Then you can call the explicitly imported functions without module name and the compiler syntactically transforms the call by adding the module name. So the compiler will transform:
map(MyFun, List) ===> lists:map(MyFun, List)
Note that this is ALL it does. There are no checks for whether the module exists or if the function is exported, it is a pure naive syntactic transformation. All it gives you is slightly shorter code. For this reason it is seldom used most people advise not to use it.
Note also that the unit of code for all operations is the module so the compiler does not do any inter-module checking or optimisation at all. Everything between modules like checking a modules existence or which functions it exports is done at run-time when you call a function in the other module.
No, there is no easy way to fix this. The source code has to be updated, and every reference to imported functions prefixed with the module in question. For example, every call to format should be replaced with io_lib:format, though you'd have to know which function was imported from which module.
You could start by removing the -import directives. The compilation should then fail, complaining about undefined functions. That is where you need to provide the correct module name. Look at the documentation pages for io_lib, erl_scan and erl_parse to see which functions are in which module.
Your problem is that you were using the experimental -import(Mod) directive which is part of parameterized modules. These are gone in R16B and onwards.
I often advise against using import. It hurts quick searches and unique naming of foreign calls. Get an editor which can quickly expand names.
Start by looking at what is stored in the location $ERL_LIBS, typically this points to /usr/lib/erlang/lib.