Databases
StackOverflow2013
Postsdbo
POWhat is the name of this monad-like functional programming pattern?
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POWhat is the name of this monad-like functional programming pattern?
    primarykey
    data
    text
    <p>I have occasionally encountered a pattern in code which resembles a monad but does not keep a consistent type across <code>&gt;&gt;=</code>.</p> <p>Here is the simplest example I could come up with:</p> <p>(First some type-level booleans:</p> <pre><code>data TyT = TyT data TyF = TyF class TyOr a b c | a b -&gt; c instance TyOr TyF TyF TyF -- rest similarly </code></pre> <p>)</p> <p>Now here is our "monad" type constructor:</p> <pre><code>data Marked p a = Marked a deriving (Show) </code></pre> <p>For a given <code>p</code>, <code>Marked p</code> is a <code>* -&gt; *</code> which acts very much like <code>m</code> in a monad, but different, as occurs next, when we define "bind":</p> <pre><code>(&gt;&gt;%) :: (TyOr p q r) =&gt; Marked p a -&gt; (a -&gt; Marked q b) -&gt; Marked r b (Marked x) &gt;&gt;% f = Marked y where Marked y = f x </code></pre> <p>What's different here is that the result of <code>&gt;&gt;%</code> has a different type constructor than the arguments. Other than that it's basically a monad.</p> <p>We could use it like this:</p> <pre><code>a :: Marked TyF Int a = Marked 5 f :: Int -&gt; Marked TyT Int f x = Marked (x + 1) ghci&gt; a &gt;&gt;% f Marked 6 ghci&gt; :t a &gt;&gt;% f a &gt;&gt;% f :: Marked TyT Int </code></pre> <p>(This was inspired by <a href="https://stackoverflow.com/questions/7131027/is-with-monadic/7131291#7131291">outis's observation that Python's "with" can't be a monad because it changes the type</a>, but I've seen it in other (simpler) ways too).</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USCommunity
    1. USOwen
    plurals
    1. PL
      singulars
      1. LTLinked
    2. PL
      singulars
      1. LTLinked
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POWhat is the name of this monad-like functional programming pattern?
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. POWhat is the name of this monad-like functional programming pattern?
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. POWhat is the name of this monad-like functional programming pattern?
      1. This table or related slice is empty.
      1. VTUpMod
    1. COThe type signature of `(>>%)` looks closer to `flip (<*>) :: Applicative f => f a -> (f a -> f b) -> f b` than `(>>=) :: Monad m => m a -> (a -> m b) -> m b`, so I'd say this is more applicative-like than monad-like.
      singulars
      1. POWhat is the name of this monad-like functional programming pattern?
      1. UShammar
    2. COYou're right. I actually want to fix that to be more monad-like though, even though I'm not really "using" monads. I'll edit it. Though I would have more or less the same question about Applicatives.
      singulars
      1. POWhat is the name of this monad-like functional programming pattern?
      1. USOwen
    3. COGoogle for "Indexed Monad", "Monadish", or "Monadic Region".
      singulars
      1. POWhat is the name of this monad-like functional programming pattern?
      1. USApocalisp
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload