How to map a 0-argument JavaScript function in PureScript FFI

Question

I am trying to import the following JavaScript function into PureScript using the FFI:

function getGreeting() {
  return "Hi, welcome to the show."
}

but I am not sure what the type should be. The closest I get to is something like:

foreign import getGreeting :: Unit -> String

I do want getGreeting to stay a function, and not convert it to a constant.

Is there a better way to write the type? I tried to see what PureScript does if I define a dummy function in PureScript itself with that type of signature:

var getGreeting = function (v) {
  return "Hi, welcome to the show.";
};

Is there a way to get rid of that v parameter that is not being used?

TIA

"I do want getGreeting to stay a function, and not convert it to a constant" I think that this function is a constant. Why you want to have such a function? — paluh
– paluh, Commented Jul 19, 2017 at 10:07
If you are using some kind of side effect or you are relying on some external state to produce or fetch this value, I think that you can set this function type to getGreeting :: forall eff. Eff (SOME_EFFECT | eff) String . — paluh
– paluh, Commented Jul 19, 2017 at 10:18
@paluh I do not want to change getGreeting because it is not my source code and should be considered unchangeable for now :) Also there are no side effects, so having the Eff type is undesirable. — Rouan van Dalen
– Rouan van Dalen, Commented Jul 19, 2017 at 11:59

gb. · Accepted Answer · 2017-07-19 17:19:47Z

3

Unit -> String is a perfectly good type for that, or perhaps forall a. a -> String. The latter type may seem too permissive, but we know for sure that the a is unused thanks to parametricity, so that the function still must be constant.

answered Jul 19, 2017 at 17:19

gb.

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paluh · Accepted Answer · 2017-07-20 07:59:48Z

3

There is really useful packagepurescript-functions which can be helpful in such a situation and if you really have to call this function from Purescript as it is (because I think that it IS really just a constant) you can try:

module Main where

import Prelude
import Control.Monad.Eff (Eff)
import Control.Monad.Eff.Console (CONSOLE, log)
import Data.Function.Uncurried (Fn0, runFn0)

foreign import getString ∷ Fn0 String

main :: forall e. Eff (console :: CONSOLE | e) Unit
main = do
  log (runFn0 getString)

I've created this simple javascript module so this example can be tested:

/* global exports */
"use strict";

// module Main

exports.getString = function() {
  return "my constant string ;-)";
};

edited Jul 20, 2017 at 7:59

answered Jul 19, 2017 at 16:52

paluh

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2 Comments

Rouan van Dalen Over a year ago

thanks paluh. It is good to know about Data.Function.Uncurried. I decided to go with the answer from gb because for something so simple, giving the type Unit -> String seems like the simplest solution.

paluh Over a year ago

Cool! Data.Function.Uncurried can be especially useful when you have functions WITH (multiple) arguments on the javascript side and don't want to wrap them to write FFI bindings ;-)

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