string - Haskell Lambda fold -


i have following algebraic data type representing lambda calculus in haskell:

data lexpr     = var string         -- variable     | app lexpr lexpr    -- function application     | lam string lexpr   -- lambda abstraction      deriving (eq, show)

i trying build accompanying fold function. acquainted general fold form algebraic data types, can in such way present: 

foldr :: (α -> β-> β) -> β -> [α] -> β foldr (#) z = go       go []     = z     go (x:xs) = x # go xs

so, have done far:

lfold :: (string -> a) -> (a -> -> a) -> (string -> -> a) -> lexpr ->   --given definition lfold f z = go       go (var _) = z            --not sure here how base case     go (lam v e) = v f go e

is correct way? if not, wrong , how fix it?

i provide hint.

suppose have list of integers type follows:

data list = nil | cons int list

then, fold becomes

foldr :: β -> (α -> β -> β) -> [α] -> β foldr nil cons = go       go nil         = nil     go (cons x xs) = cons x (go xs)

notice that, once name arguments nil, cons it's matter of 1) mapping nil (constructor) nil (parameter), 2) mapping cons cons, 3) applying go subterms of type list (i.e., xs).

for type,

data lexpr     = var string         -- variable     | app lexpr lexpr    -- function application     | lam string lexpr   -- lambda abstraction

we can use same trick:

lfold :: (string -> a) -> (a -> -> a) -> (string -> -> a) -> lexpr ->    lfold var app lam = go       go (var v)     = ??     go (app e1 e2) = ??     go (lam v e)   = ??

notice how named 3 arguments: var, app, lam. checking happened in list type above, should able fill in blanks.


Comments

Post a Comment

Popular posts from this blog

c# - Validate object ID from GET to POST -

i have method allows user access comment have made on site. have post method lets them update comment. my normal solution send entire comment model through, let them update it, update on database when post back. involve sending commentid through in hiddenfor , can manipulated. how can verify commentid sent in get method same i'm getting in post , not able alter comment wish? how can verify commentid sent in method same i'm getting in post? basically have validate following things in post - user logged-in user. authenticated users post comments. commentid in post , should valid commentid , should present in database. userid associated logged-in user should same userid associated comment . comment should contain userid column, can check @ time of update. to make sure update happens comment has been sent in get - hold commentid in session , in post action compare commentid value in session .
Read more

node.js - Custom Model Validator SailsJS -

when using custom model validator checks record within model. return within spread doesn't seem end control flow. //post.js model /** * custom validator **/ types: { isuservalid: function(user_id) { var promise = require('bluebird'); promise.all([ user.findone({id: user_id}) ]) .spread(function(user) { console.log(user); if (user === null || user === undefined) { console.log('failed'); return false; }else{ console.log('passed'); return true; } }); } }, my response standard validation failed response. { "error": "e_validation", "status": 400, "summary": "1 attribute invalid", "model": "post", "invalidattributes": { "owner": [ { "rule": "isuservalid", ...
Read more

php - Find a regex to take part of Email -

i have following emails need extract part from: bestellnummer xxx: 1 von xxx, 1er pack ------------- anfang der nachricht ------------- foo bar baz foo bar baz // <<<<< need text here ------------- ende der nachricht ------------- ------------- anfang der nachricht ------------- foo bar baz foo bar baz ------------- ende der nachricht ------------- there 0 unlimited occurences of ------------- anfang der nachricht ------------- ------------- ende der nachricht ------------- and i'm able extract first part regex: $re = "/------------- .*? -------------.?(.*?).?------------- .*? -------------/s"; but, i'm quite new on learning regex, i'm pretty sure there must better regex extract part (foo bar baz foo bar baz) of text between ------------- anfang der nachricht ------------- ------------- ende der nachricht ------------- as can in different languages, i'm using .? to match between hyphens. i...
Read more