haskell - Exporting a polymorphic MonadState function for a particular state data type -


what i'm trying (in module i'm writing) export function works on particular type in state monad (in example below, type foo). user able use function in whatever monadstate type wish: state.lazy, state.strict, statet, etc. needs polymorphic in outer state monad.

here example of i'd do:

edited better question:

import control.monad.state  data foo = foo { cnt :: int, val :: }  --test :: state (foo a)           --  works --test :: statet (foo a) maybe    --  ...so --  ... including following signature gives error: test :: monadstate (foo a) m => m test = modify (\(foo a)-> foo (i+1) a) >> gets val

ghc complains flexibleinstances extension required define type above. using extension correct way define function or there better way?

thanks

can't use monadstate typeclass?

{-# language flexiblecontexts #-} import control.monad.state  data foo = foo { cnt :: int, val :: }   test :: monadstate (foo a) m => m test = modify (\(foo a)-> foo (i+1) a) >> gets val

it loads fine in ghci.

edit: mtl-2.0 , ghci-7.0.1


Comments

Post a Comment

Popular posts from this blog

asp.net - repeatedly call AddImageUrl(url) to assemble pdf document -

i'm using abcpdf , i'm curious if can recursively call addimageurl() function assemble pdf document compile multiple urls? something like: int pagecount = 0; int theid = thedoc.addimageurl("http://stackoverflow.com/search?q=abcpdf+footer+page+x+out+of+", true, 0, true); //assemble document while (thedoc.chainable(theid)) { thedoc.page = thedoc.addpage(); theid = thedoc.addimagetochain(theid); } pagecount = thedoc.pagecount; console.writeline("1 document page count:" + pagecount); //flatten document (int = 1; <= pagecount; i++) { thedoc.pagenumber = i; thedoc.flatten(); } //now try again theid = thedoc.addimageurl("http://stackoverflow.com/questions/1980890/pdf-report-generation", true, 0, true); //assemble document while (thedoc.chainable(theid)) { thedoc.page = thedoc.addpage(); theid = thedoc.addimagetoc...
Read more

java - Android recognize cell phone with keyboard or not? -

in android app, want detect user's device if device has keyboard(like motorola milestone) , show corresponding user interface. guys know how that? ! regards justicepenny http://developer.android.com/reference/android/content/res/configuration.html public int keyboard the kind of keyboard attached device. 1 of: keyboard_nokeys, keyboard_qwerty, keyboard_12key. that's it!
Read more

iphone - How would you achieve a LED Scrolling effect? -

how achieve led scrolling effect example below? led sign - led ticker emulator iphone , ipad http://img.skitch.com/20101201-rsfh4p1bajb1wp94k466pdpiuj.preview.jpg sometimes remunerates comment in other posts :) have put code you, should implement font '8pin matrix' realistic feeling. uilabel *label; - (void)viewdidload { [super viewdidload]; label = [[uilabel alloc]initwithframe:cgrectmake(10, 50, 300, 20)]; label.font = [uifont fontwithname:@"courier-bold" size:16.0]; label.backgroundcolor = [uicolor blackcolor]; label.textcolor = [uicolor greencolor]; label.linebreakmode = uilinebreakmodecharacterwrap; // otherwise "…" label.text = @"this led text scrolled "; // blanks space between [self.view addsubview:label]; [nstimer scheduledtimerwithtimeinterval:0.12f target:self selector:@selector(scrolllabel) use...
Read more