generics - Very weird type mismatch in Scala -


why error when firing printgenerictype(new box[master]()) , , not error in case if run printgenerictype(new box()) on new box[master]()?

object varianceexample extends app {    new box[master]().printgenerictype(new box()) // ok   new box().printgenerictype(new box[master]()) // fail  }  class box[t >: tool](implicit m: manifest[t]) {   def printgenerictype(box: box[t]) = {     println(s"generic type [$m] - $box")   } }  class master class tool   extends master class hammer extends tool

you need define type t covariant, otherwise can pass same type used in constructor. new box()- gives box[tool] (without other constraints) , trying pass box[master].

in first example scalac automatically infers new box() box[master] because it's in box[master] position.

not sure want achieve, solve concrete problem need define type box this:

class box[+t >: tool](implicit m: manifest[t]) {   def printgenerictype[a >: t](box: box[a]) = {     println(s"generic type [$m] - $box")   } }

Comments

Post a Comment

Popular posts from this blog

resizing Telegram inline keyboard -

telegram's inline keyboard great feature lots of different use cases. inline buttons added list of items this: inline_keyboard = [[inlinekeyboardbutton(text="button", callback_data="button"), inlinekeyboardbutton(text="reset",callback_data="reset")]] inline_keyboard_markup = inlinekeyboardmarkup(inline_keyboard) update.message.reply_text("hi", reply_markup=inline_keyboard_markup) the above code adds 2 buttons each half width of chat screen. i know the normal keyboard button there resize_keyboard parameter somehow can used. my question is there way resize inline buttons ? example make full width or quarter width. unfortunately, can't now. :( you can suggest @botsupport , might add feature next version.
Read more

command line - How can a Python program background itself? -

consider following python command-line program. doesn't work because incomplete, think illustrates problem: #!/usr/bin/python3 import multiprocessing pool = multiprocessing.pool() task in expensive_iterator_with_user_interaction(): pool.call_async(expensive_computation, (task,)) pool.close() print("dear user, may press ctrl-z , bg script.") pool.join() thus, problem scripts needs user input while it's launching expensive parallel processes. there moment when user interaction not needed anymore. then, ask user background process shell actions. however, can achieved program itself? (side note: core of problem entanglement of user input firing “expensive computations”. therefore, cannot separate both in 2 scripts, , letting 1 spawning other daemon.) the command screen may you. use screen -l create new session , run application. after inputting data, can press " ctrl-a " , press " d " on keyboard detach session. now...
Read more

php - "cURL error 28: Resolving timed out" on Wordpress on Azure App Service on Linux -

Image
my wordpress installation on azure app service on linux using custom docker container has slow response times. pages take around 20-40 seconds load. i have troubleshooting plugin installed indicates problem "curl error 28: resolving timed out after n milliseconds" when making requests following urls https://api.wordpress.org/core/version-check/1.7/ http://api.wordpress.org/core/version-check/1.7/ https://api.wordpress.org/plugins/update-check/1.1/ http://api.wordpress.org/plugins/update-check/1.1/ https://api.wordpress.org/themes/update-check/1.1/ http://api.wordpress.org/themes/update-check/1.1/ curl works fine on scm-site command line. example works ok. curl -x post http://api.wordpress.org/core/version-check/1.7/ edit if ssh container , run php code, works fine. <?php $url = 'http://api.wordpress.org/core/version-check/1.7/'; $fields = array( 'version' => urlencode('4.8.1'), 'php' => urlencode(...
Read more