functional programming - Is there a way in Haskell to express a point free function to edit the property of a data type? -

-

i have type:

data cons = cons {myprop :: string}

and later on i'm mapping on list, setting property different value:

fmap (\x -> x{myprop = ""}) mylist

is there point free way express anonymous function?

there isn't point free way of doing this, can instead use lens library (which comes whole basket of goodies):

{-# language templatehaskell #-} import control.lens  -- if put _ @ beginning of field name lens can derive methods data cons = cons { _myprop :: string } deriving (eq, show) -- derives myprop lens (works fields in record) makelenses ''cons -- can manually -- myprop :: functor f => (string -> f string) -> cons -> f cons -- myprop = lens _myprop (\c newprop -> c { _myprop = newprop }) -- lets define lenses type without depending on lens -- bigger deal might think.  can support library , -- , paradigms without depending on @ all.  main =     let cs = map cons ["a", "b", "c"]         -- prefix set function         test1 = fmap (set myprop "") cs         -- or operator         test2 = fmap (myprop .~ "") cs     print cs     print test1     print test2

that "basket of goodies" comes lenses contains things like

data email = email     { _emailaccount :: string     , _emaildomain :: string     } deriving (eq, show) makelenses ''email  data person = person     { _personname :: string     , _personage :: int     , _personemail :: email     } deriving (eq, show) makelenses ''person  testpeople :: [person] testpeople = [     person "a" 40 (email "aaa" "gmail.com"),     person "b" 45 (email "bbb" "email.com"),     person "c" 50 (email "ccc" "outlook.com")]  domains :: [person] -> [string] domains ps = ps^..traverse.personemail.emaildomain  statefulfun :: monadio m => statet person m () statefulfun =     liftio $ putstrln "changing name"     personname .= "a different name"     liftio $ putstrln "the name changed!"     personemail.emailaccount %= map toupper  morestate :: monadio m => statet person m () morestate =     personname .= "foo"     zoom personemail $         current <-         liftio $ putstr "current email is: "         liftio $ print current         emailaccount .= "foo"         emaildomain  .= "foo.com"  main :: io () main =     finalstate <- execstatet (morestate >> statefulfun) (head testpeople)     print finalstate

as can see, lenses compose backwards (they don't, it's because have more general type lets them crazy things). there's nice ways of traversing complex data structures, optionally performing effects go, , plenty of operators writing imperative looking stateful code appears normal oop method access dot. lets abstract common , complex patterns on large , complex data structures little effort, once start grok them @ least!


Comments

Post a Comment

Popular posts from this blog

c++ - OpenCV Error: Assertion failed <scn == 3 ::scn == 4> in unknown function, -

-
i read alot other solution still confused should mine... #include <opencv2/objdetect/objdetect.hpp> #include <opencv2/highgui/highgui.hpp> #include <opencv2/imgproc/imgproc.hpp> #include <iostream> #include <stdio.h> using namespace std; using namespace cv; int main(int argc, const char** argv) { //create cascade classifier object used face detection cascadeclassifier face_cascade; //use haarcascade_frontalface_alt.xml library face_cascade.load("haarcascade_frontalface_alt.xml"); //setup video capture device , link first capture device videocapture capturedevice; capturedevice.open(0); //setup image files used in capture process mat captureframe; mat grayscaleframe; //create window present results namedwindow("outputcapture", 1); //create loop capture , find faces while (true) { //capture new image frame capturedevice >> captureframe; ...
Read more

php - render data via PDO::FETCH_FUNC vs loop -

-
i've been using 2 methods render data. the first one: function name($id,$name){ return '<div id="'.$id.'">'.$name.'</div>'; } echo implode($pdo->query("select id,name user")->fetchall(pdo::fetch_func,'name')); the second one: $users = $pdo->query("select id,name user")->fetchall(pdo::fetch_obj); foreach($users $user){ echo name($user->id,$user->name); } i don't understand how pdo::fetch_func works. tried figure out. however, not well-documented. could please explain fetch mode? , also, 1 performs better? thank you. both methods wrong , have learn how use templates , how separate business logic presentation logic. $users = $pdo->query("select id,name user")->fetchall(pdo::fetch_obj); tpl::assign('users', $users); is code business logic part. then in template <?php foreach $users $row): ?> <div id="<?=$...
Read more

The canvas has been tainted by cross-origin data in chrome only -

-
uncaught securityerror: failed execute 'getimagedata' on 'canvasrenderingcontext2d': canvas has been tainted cross-origin data i getting above error in chrome. code(following) works fine in mozilla. have not corss domain issue. why above error in chrome? var wheel_canvas, wheel_context, can_w, can_h, wheel_grd; var large_radius = 160; var small_radius = 120; var wheel_canvas = document.getelementbyid('wheel_canvas'); var wheel_context = wheel_canvas.getcontext('2d'); var can_w = $(wheel_canvas).width(); var can_h = $(wheel_canvas).height(); var centerx = can_w / 2, centery = can_h / 2; var wheel_grd = null; test('#ff0000'); function test(hex) { $('#clrpicker').css({'left': 210, 'top': 210 }); inverted = hexbw(hex); $('#color_val_show').val(hex); current_color_hex_val_selected = hex; $('#color_val_show').css({'color':inverted,'background':hex}); fillgradient...
Read more