pipe-first.md

title	Pipe First

BuckleScript has a special |. (or -> for Reason) pipe syntax for dealing with various situations. This operator has many uses.

Pipelining

The pipe takes the item on the left and put it as the first argument of the item on the right. Great for building pipelines of data processing:

a
|. foo b
|. bar

a
->foo(b)
->bar

is equal to

bar(foo a b)

bar(foo(a, b))

JS Method Chaining

JavaScript's APIs are often attached to objects, and often chainable, like so:

constresult=[1,2,3].map(a=>a+1).filter(a=>a%2===0);asyncRequest().setWaitDuration(4000).send();

Assuming we don't need the chaining behavior above, we'd bind to each case this using bs.send from the previous section:

externalmap : 'aarray -> ('a -> 'b) -> 'barray="map" [@@bs.send]
externalfilter : 'aarray -> ('a -> 'b) -> 'barray="filter" [@@bs.send]
typerequestexternalasyncRequest: unit -> request="asyncRequest"externalsetWaitDuration: request -> int -> request="setWaitDuration" [@@bs.send]
externalsend: request -> unit="send" [@@bs.send]

[@bs.send] externalmap: (array('a),'a => 'b) => array('b) ="map";
[@bs.send] externalfilter: (array('a),'a => 'b) => array('b) ="filter";typerequest;externalasyncRequest:unit => request="asyncRequest";
[@bs.send] externalsetWaitDuration: (request,int) => request="setWaitDuration";
[@bs.send] externalsend:request => unit="send";

You'd use them like this:

let result = filter (map [|1; 2; 3|] (funa -> a +1)) (funa -> a mod2=0)
let()= send(setWaitDuration (asyncRequest()) 4000)

letresult= filter(map([|1,2,3|], a => a +1), a => a mod2==0);
send(setWaitDuration(asyncRequest(),4000));

This looks much worse than the JS counterpart! Now we need to read the actual logic "inside-out". We also cannot use the |> operator here, since the object comes first in the binding. But |. and -> work!

let result = [|1; 2; 3|]
|. map(funa -> a +1)
|. filter(funa -> a mod2==0)
let()= asyncRequest ()|. setWaitDuration 400|. send

letresult=[|1,2,3|]->map(a => a +1)
->filter(a => a mod2===0);
asyncRequest()->setWaitDuration(400)->send;

Pipe Into Variants

This works:

let result = name |. preprocess |.Some

letresult= name->preprocess->Some

We turn this into:

let result =Some(preprocess(name))

letresult=Some(preprocess(name))