elm - Converting A Maybe Value Into A Normal Value -


i have no clue how convert maybe value normal value ...

i have these lines...

pickchord : model -> note -> chord pickchord model note =     let         nextchord = list.head (list.filter (testchord note) model.possiblemajorchords)     in         nextchord

the compiler complains:

the definition of pickchord not match type annotation. - type annotation pickchord says returns:

chord

but returned value (shown above) a:

maybe chord

how handle problem?

thanks.

this why love elm. in it's own way, elm telling you've got design flaw.

let's take closer look. here's code as-is:

pickchord : model -> note -> chord pickchord model note =     let         nextchord = list.head (list.filter (testchord note) model.possiblemajorchords)     in         nextchord

so you've got list of major chords filter specific note. produces list of chords. however, list type can represent empty list, why list.head returns maybe. in case, list.head can return either nothing or just chord.

now, can work around default values , error handling, work-arounds dodge real problem: the list type doesn't accurately fit problem domain.

if you've got list of all major chords, can't think of reason why ever end empty list after applying filter. filter should find @ least 1 chord. assuming that's case, need list-like type can represent list never empty; means head return chord , not maybe chord. better representation of you're trying achieve. luckly, there's such type (which didn't create use extensively) called list.nonempty. here's how work:

import list.nonempty ne  pickchord : model -> note -> chord pickchord model note =             ne.head (ne.filter (testchord note) model.possiblemajorchords)

your model.possiblemajorchords have change list chord list.nonempty chord, makes whole maybe problem go away. of course assuming claim filter returning @ least 1 chord holds true.

here's link nonempty package: http://package.elm-lang.org/packages/mgold/elm-nonempty-list/latest


Comments

Post a Comment

Popular posts from this blog

PHP and MySQL WP -

i trying accomplish 2 things here code. writing php directly wp page using insert php plugin. 1) query mysql database , return results 1 column 4 columns. 2) make each result own hyperlink directs user new wp page runs new query on page. i able query show results , turn results dynamic hyperlink, cannot formatting down. right returns result 1 column. here have: [insert_php] $servername = "localhost"; $username = "login"; //edited privacy $password = "password"; //edited privacy $database = "database"; //edited privacy // create connection $conn = new mysqli($servername, $username, $password, $database); // check connection if ($conn->connect_error) { die("connection failed: " . $conn->connect_error); } echo "connected successfully"; //get results database $result = mysqli_query($conn,"select mine mines order mine"); $all_property = array(); //declare array saving property //showing proper...
Read more

android - InAppBilling registering BroadcastReceiver in AndroidManifest -

i'm implementing inapp billing in android app. works well, however, i'm trying decouple broadcast receiver activity manifest. suggestion in android's trivialdrive sample: // important: dynamically register broadcast messages updated purchases. // register receiver here instead of <receiver> in manifest // because call getpurchases() @ startup, therefore can ignore // broadcasts sent while app isn't running. // note: registering listener in activity bad idea, done here // because sample. regardless, receiver must registered after // iabhelper setup, before first call getpurchases(). currently there's class extends broadcastreceiver : public class iabbroadcastreceiver extends broadcastreceiver { /** * intent action receiver should filter for. */ public static final string action = "com.android.vending.billing.purchases_updated"; private final iabbroadcastlistener mlistener; public iabbroadcastreceiver(iabbroadcastlistener listener) { m...
Read more

nginx - phpPgAdmin - log in works but I have to login again after clicking on any links -

i have setup postgres 9.6 , phppgadmin on centos 7 server nginx, i can log in phppgadmin, postgresql server on right of page remains crossed out , link try browse (databases, roles etc) requires log in again. i'm thinking seems sessions problem, unsure how resolve it. any pointers on how fix or should looking @ in order track down problem? solved. turns out owner of /var/lib/php/session apache - needed nginx. sorted changing ownership nginx below: chown -r nginx:nginx /var/lib/php/session/
Read more