haskell - Types and do notation -


this several questions rolled one:

  1. in do notation, each line have return same type? example, can write 1 line in single do block returns io monad, , returns integer? (my understanding, based on how de-sugaring >> , >>= seems work, answer no.)

  2. if not, how compiler determine type lines must return? in examples i've seen, author takes foregone conclusion we're working io monads. how know, given do block, each line must return?

  3. again assuming answer #1 no: how use functions don't return right kind of monad inside do block? example, consider websockets code:

    application :: mvar serverstate -> ws.request -> ws.websockets ws.hybi00 () application state rq =   ws.acceptrequest rq   msg <- ws.receivedata :: ws.websockets ws.hybi00 text   return ()

    suppose want print value of msg. how go in way doesn't conflict type of do block?

  1. in do-block, each line can return different types, must in same monad
    • one line can return io string , 1 can return io integer both have io.
  2. the same rest of haskell. type inference. in rest of haskell doesn't work, , when doesn't must annotate well.
  3. there 2 ways of doing
    • let, remember how in ghci must use let declare local variables, can same in do block. let somemonad = dosomething
      • notice, no in
    • monad transformers! big topic explain in blurb, they're monads have special function lift can "lift" monad transformer. transformers end in t, eg statet. pretty every monad use has equivalent transformer.

Comments

Post a Comment

Popular posts from this blog

jquery - How can I dynamically add a browser tab? -

since using iframes house external pages are, accounts "not preferred method" , disallowed sites (flickr, one), want dynamically generate browser tabs. e.g., when user submits form, want dynamically generate tab on browser in response submitted. e.g., might want add new tab url like: http://www.bigsurgarrapata.com/contact how in jquery? you want form target attribute action attribute. http://www.w3schools.com/tags/att_form_target.asp browsers have ultimate control on result of target attribute open new tab value of _blank. <form action="contact_submit.php" target="_blank" method="post">
Read more

keyboard - C++ GetAsyncKeyState alternative -

i using windows.h library's getasynckeystate function define key pushed on keyboard . here program returns int value of pushed key example. #include <iostream> #include <windows.h> using namespace std; int main (){ char i; for(i=8;i<190;i++){ if(getasynckeystate(i)== -32767){ cout<<int (i)<<endl; } } return 0; } but trying make opengl simple game , function appeared slow . there function not need cycle 180 times , if statement . thanks you're trying make opengl game, you're going use glfw. if that's case can away using glfwgetkey() such: if (glfwgetkey(glfw_key_up) == glfw_press) {...} if (glfwgetkey(glfw_key_down) == glfw_press) {...} that method nicely explained in tutorial 6 opengl-tutorial.org . should start first tutorial of beginners tutorials , later intermediate tutorials . if you're going make win32 window can listen wm_keydown or wm_keyup messages , virtua...
Read more

android - java.net.UnknownHostException(Unable to resolve host “URL”: No address associated with hostname) -

i developing app testing asp.net webservice in android.for simple webservice adding 2 numbers used.where numbers passed parameters.the result in dialogue.the webserver works in local. when application runs following result exception.i provided internet permission in manifest. java.net.unknownhostexception(unable resolve host “ http://url.com/ ”: no address associated hostname) my code given below main activity public static string rslt=""; /** called when activity first created. */ @override public void oncreate(bundle savedinstancestate) { super.oncreate(savedinstancestate); setcontentview(r.layout.activity_main); button b1=(button)findviewbyid(r.id.button1); final alertdialog ad=new alertdialog.builder(this).create(); b1.setonclicklistener(new onclicklistener() { @override public void onclick(view arg0) { // todo auto-generated method stub try { ...
Read more