Execute Fluture task in middle of Sanctuary pipe

Question

I have a pipe like this:

S.pipe([
    getRequestFile,                  // not async
    S.chain(saveTemporary),          // not async
    S.chain(copyImageToPublicPath),  // async
    S.chain(copyFileToPath),         // async
    S.chain(someLoggingFunction),    // not async
    S.chain(sendResponse),           // not async
]);

There are 2 async functions in middle of this pipe. I want to await for copyImageToPublicPath and then await for copyFileToPath and then continue the normal flow

After some search I found that there is Future.tryP function for doing async but how can I use Fluture in middle of this pipe?

Answer 1

It's a matter of lining up the types.

Let's make up some type definitions to use in an example:

foo :: String -> String
bar :: String -> Future Error String
baz :: String -> Array String

Now, let create our program step by step…

//    program :: a -> a
const program = S.pipe ([
]);

//    program :: String -> String
const program = S.pipe ([
  foo,               // :: String
]);

//    program :: String -> Future Error String
const program = S.pipe ([
  foo,               // :: String
  bar,               // :: Future Error String
]);

//    program :: String -> Future Error (Array String)
const program = S.pipe ([
  foo,               // :: String
  bar,               // :: Future Error String
  S.map (baz),       // :: Future Error (Array String)
]);

To operate on the b inside a Future a b value we use either S.map or S.chain.

S.map can lead to unwanted nesting:

fut :: Future Error String

quux :: String -> Future Error Number

S.map (quux) (fut) :: Future Error (Future Error Number)

We could use S.chain to avoid this nesting:

fut :: Future Error String

quux :: String -> Future Error Number

S.chain (quux) (fut) :: Future Error Number

It may be helpful to think of S.map adding to some computation an operation that cannot fail, whereas S.chain adds a computation that can fail.