Haskell List concatenation Inferred Type -


trying replace element within list @ given point new element return element.

 setelt :: int -> [a] -> -> [a]  setelt x (yf:(y:yl)) z    | x == (length yf) = (yf:(z:yl))

results in error:

inferred type not general enough expression    : setelt expected type : int -> [a] -> -> [a] inferred type : int -> [[a]] -> [a] -> [[a]]

doesn't seem have problem concatenation of yf:y:yl not sure how solve.

you seem misunderstanding list constructor (:) does. haskell list sequence of (:) constructors ending empty list [], , pattern matching disassembles constructors in same order.

so when pattern match on (yf:(y:yl)) you're matching list of @ least 2 elements, yf , y, , yl rest of list.

the inferred type not expect because length yf implies yf--which first element of input list--is list.

what need instead walk down list recursively, building new list using either current element of input or replacement element x when reach right location. general form should standard library function map, implemented this:

map _ [] = [] map f (x:xs) = f x : map f xs

except you'll need way track index you're searching for, rather transforming every element.

your current function fail if applied list of 0 or 1 elements, fixing should easy after correcting algorithm whole.


Comments

Post a Comment

Popular posts from this blog

Why does Ruby on Rails generate add a blank line to the end of a file? -

running rails generate controller foo home results in: class foocontroller < application controller def home end end (there's nothing on line in actual file, blank line) is there purpose blank line? it's common use newline indicate end of line, last line. many editors (like vi, use) add in newline silently. consensus text files (especially in unix world) should end in newline, , there have been problems historically if wasn't present. why should text files end newline? the tool used count lines in file "wc" counts newlines in file, without trailing newline show 3 instead of 4. it improves readability of templates used in generator. consider: https://github.com/rails/rails/blob/master/railties/lib/rails/generators/rails/controller/templates/controller.rb to remove trailing newline, template have last line of: end<% end -%> instead of: end <% end -%> that seems less readable me. the discussion below refers ...
Read more

keyboard - Smiles and long press feature in Android -

i've created android keyboard. how add features inserting smilies, , using long keypresses display symbols? to create long-press behavior allows select several options pop-up menu, follow guide (specifically pop-up menu section): http://developer.android.com/guide/topics/ui/menus.html smileys (also known emoticons) combination of keys, understood programs , parsed same way. have do, when clicks smiley face on keyboard, simulate pressing of key combination matching emoticon. example, when clicks smiley, should simulate key presses ':' , ')' :). here's list of emoticons: http://en.wikipedia.org/wiki/list_of_emoticons
Read more

node.js - Bad Request - node js ajax post -

i'm using express 3.0 , node v0.11.0. have button submits form , i'm using ajax post data json object node server. client code is: $('#contact').submit(function(e) { console.log('submit called'); var formdata = json.stringify($('form').serializeobject()); console.log(formdata); $.ajax({ url: "http://localhost:3000/savedata/", type: "post", datatype: 'json', data: {objectdata: formdata}, contenttype: "application/json", complete: function() { console.log('process complete'); }, success: function(data) { console.log('process sucess'); }, error: function() { console.log('process error'); }, }); return false; }); then on server: var express = require('express'); var app = express(); app.set('views', __dirname + '/views'); app.set('view engine', 'jade'); app.use(express.bodyparser());...
Read more