lifting into a data type (Haskell) -

- 

type pt_int = int type pt_string = string data polytype = pt_int int | pt_string string

given function f, how write function lifts polytype? (just trying understand lifting)

your polytype equivalent either int string. in case haven't seen either before:

data either b = left | right b

so have function like

liftp :: (either int string -> a) -> polytype -> liftp f poly = case poly of     pt_int    -> f (left i)     pt_string s -> f (right s)

polytype contains either int or string, can lift functions defined on both int , string.

that said, don't think you're after. term "lifting" used in context of polymorphic data types [a], maybe a, (->) a or in general type f a f :: * -> *.

in these cases, given function g :: -> b, want new function [a] -> [b], maybe -> maybe b or in general f -> f b. fmap functor.

class functor f     fmap :: (a -> b) -> (f -> f b)

but polytype monomorphic (it doesn't have free variable in type. precise, has kind *) can't functor.

you chould change definition of polytype to

data polytype = pt

now valid functor (it's identity functor)

instance functor polytype     fmap f (pt a) = pt (f a)

the type of fmap (specialized particular polytype instance) is

fmap :: (a -> b) -> polytype -> polytype b

Comments

Post a Comment

Popular posts from this blog

c# - Unity IoC Lifetime per HttpRequest for UserStore -

-
i'm trying clean default implementation of accountcontroller.cs comes out of box in new mvc5/owin security implementation. have modified constructor this: private usermanager<applicationuser> usermanager; public accountcontroller(usermanager<applicationuser> usermanager) { this.usermanager = usermanager; } also, have created lifetime manager unity looks this: public class httpcontextlifetimemanager<t> : lifetimemanager, idisposable { private httpcontextbase _context = null; public httpcontextlifetimemanager() { _context = new httpcontextwrapper(httpcontext.current); } public httpcontextlifetimemanager(httpcontextbase context) { if (context == null) throw new argumentnullexception("context"); _context = context; } public void dispose() { this.removevalue(); } public override object get...
Read more

Change the color of an oval at click in Java AWT -

-
i have draw ovals in java, , @ click change color. beginning tried change color after 20 ms, doesn't work. my code is: public class mycomponentnew extends frame { public graphics2d g2d; public mycomponentnew(string title) { super(title); setsize(400, 550); } @override public void paint(graphics g) { this.g2d = (graphics2d) g; this.g2d.setcolor(color.red); this.g2d.filloval(10, 55, 50, 100); } public void changecolor () { this.g2d.setcolor(color.blue); this.g2d.filloval(10, 55, 50, 100); } } and in class main method have: mycomponentnew m; m = new mycomponentnew("fereastra cu baloane"); m.setvisible(true); m.addwindowlistener(new windowadapter() { @override public void windowclosing(windowevent we) { system.exit(0); } }); try { thread.sleep(20); } catch(interruptedexception e) {} m.changecolor(); the color of oval remains red. ...
Read more

I am trying to solve the error message 'incompatible ranks 0 and 1 in assignment' in a fortran 95 program. -

-
i writing program in fortran find velocity of parachuting person in relation time. keep getting error can't fix. new programming , appreciated. the error is v(i+1)=v(i)+[32-((c*v(i)*v(i))/m)]*(h) 1 error: incompatible ranks 0 , 1 in assignment @ (1) and program is program para integer :: real :: v(11) !velocity real :: q !initial velocity real :: h !time step real :: c !drag coefficient real :: m !mass ! gravity equal 32 ft/s^2 write (*,*)'enter time step' read(*,*)h write(*,*)'enter initial velocity' read(*,*)q write(*,*)'enter drag coefficient' read(*,*)c write(*,*)'enter mass' read(*,*)m i=1,10 ! 1 10, 1 being interval. end v(i+1)=v(i)+[32-((c*v(i)*v(i))/m)]*(h) q=v(1) end program you cannot use [] normal parenthesis in expressions. array constructor, [ items ] means array items elements. end do should after line.
Read more