haskell - What "Contraint is no smaller than the instance head" means and how to solve it -


i write like:

{-# language flexiblecontexts,flexibleinstances #-}  import data.bytestring.char8 (bytestring,pack) import data.foldable (foldable)  class (show a) => rows     rrepr :: -> [bytestring]     rrepr = (:[]) . pack . show  instance (foldable f,show (f a)) => rows (f a)     rrepr = const []

meaning f a instantiate rows if f instantiate foldable , f a instantiate show. when run ghc get:

constraint no smaller instance head   in constraint: show (f a) (use -xundecidableinstances permit this) in instance declaration `rows (f a)'

i have 2 questions:

  • what "smaller" means in error , problem?
  • what right way define want without using undecidableinstances?

let's play compiler: have type (f a) we'd see if valid satisfier of rows constraint. so, need dispatch proof (f a) satisfies show (f a) well. isn't problem unless writes an

 instance rows (f a) => show (f a) ...

in case i'm began. encoding infinite loop sort of foolish, haskell statically ensures it's impossible unless ask undecidableinstances.

normally haskell ensures each step of "resolution chase" reduces size of type @ least 1 constructor. leads simple proof structural induction we'll end bare type in finite number of resolutions.

this overly restrictive, instance resolution steps meaningful, useful, , total if don't reduce constructor tree. same kind of totality restriction applies in languages agda , coq , it's bit of illustrative challenge manipulate algorithm 1 proceeds structural induction.

so how can fix it? 1 way lose show constraint on class definition use instance show1 prelude-extras. 

class show1 f ... show1 :: (show1 f, show a) => f -> string  -- not instance definition!

and have instance (foldable f, show1 f, show a) => rows (f a) ... works in testing. can write default instance standalone function.

defrrepr :: show => -> [bytestring] defrrepr = (:[]) . pack . show

and use whenever writing instance definition showable thing.

the other solution use newtype wrappers allow haskell see "layer" of resolution has been removed.

instance (foldable f, show (f a)) => rows (foldablerow f a)     rrepr = const [] . unfr  newtype foldablerow f = fr { unfr :: f } deriving (show)

Comments

Post a Comment

Popular posts from this blog

curl - PHP fsockopen help required -

i trying work on cloudflare api , have found in documentation. unfortunately have no idea how can use in fsockopen or through curl. here example of post request. need know how can make request below data post using curl or fsockopen curl https://www.cloudflare.com/api_json.html -d 'a=cache_lvl' \ -d 'tkn=8afbe6dea02407989af4dd4c97bb6e25' \ -d 'email=sample@example.com' \ -d 'z=example.com' \ -d 'v=agg' here link cloudflare https://www.cloudflare.com/docs/client-api.html#s4.2 try one: $ch = curl_init("https://www.cloudflare.com/api_json.html"); curl_setopt($ch, curlopt_returntransfer,1); curl_setopt($ch, curlopt_followlocation, 1); curl_setopt($ch, curlopt_ssl_verifypeer,false); // handling of -d curl parameter here. $param = array( 'a' => 'cache_lvl', 'tkn' => '8afbe6dea02407989af4dd4c97bb6e25', 'email' => 'sample@example.com', 'z' =...
Read more

HTTP/1.0 407 Proxy Authentication Required PHP -

i working in restricted network/limited access internet. since have use proxy make web service call. here php code calling web service , response . $client = new soapclient("http: //www.xyz.com/abc/wsdls/login.wsdl", array("trace" => 1, "exceptions" => 1, 'proxy_host' => $proxy_host, 'proxy_port' => $proxy_port, 'proxy_login' => $proxy_login, 'proxy_password' => $proxy_password)); response is fatal error: uncaught soapfault exception: [wsdl] soap-error: parsing wsdl: couldn't load ' http://www.xyz.com/abc/wsdls/login.wsdl ' : failed load external entity if copy url error message , paste in browser(with same proxy settings) works. 2nd scenario is, if pass values instead of variables giving me diffarant response $client = new soapclient("http: //www.xyz.com/abc/wsdls/login.wsdl", array("trace" => 1, "exceptions" => 1, 'proxy_host' ...
Read more

c# - Resource not found error -

i have problem accessing method on homecontroller. show code of method : [httpget] public actionresult decriptidentifiantetredirige(string login_crypter, string mdp_crypter) { string loginacrypter = _globalmanager.protegemotdepasse(login_crypter); string mdpacrypter = _globalmanager.protegemotdepasse(mdp_crypter); user userapp = new models.user(login_crypter, mdp_crypter); if (userapp.authentificationvalidee(userapp.userlogin, userapp.password)) { session["name"] = userapp.userlogin; return redirecttoaction("accueil", "home"); } else { return redirecttoaction("validerauthentification", "home"); } } then in routeconfig.cs wrote route : routes.maproute( name: "authentificationapresdecryptage", url: "{controller}/{action}/{login_crypter}/{mdp_crypter}", defaults: new...
Read more