PureScript - Integer from Number using fromMaybe

Question

Consider the following PureScript code, which is trying to convert a Number into an Integer.

module Main where

import Prelude (discard, Unit)
import Prelude (($))
import Data.Int ( toStringAs, fromNumber, decimal )
import Data.Maybe ( fromMaybe )

myNumber :: Number
myNumber = 4.762

myInteger :: Int
myInteger = fromMaybe (0 $ (fromNumber myNumber))

However this code produces the following error,

 Could not match type
       
    Int
       
  with type
  
    t0 -> t1

Not sure how to use fromMaybe correctly here as this use case doens't seem to be documented yet.

So the question is, What is the correct syntax for converting a number to an integer?

score 2 · Accepted Answer · answered Apr 06 '22 at 15:57

It's not 100% clear what you're trying to do, but it kinda looks like you have an extra set of parens.

myInteger = fromMaybe (0 $ (fromNumber myNumber))
                      ^                         ^
                      |                         |
                       these two are detrimental

The expression inside these two parens is:

0 $ (...)

If you look at the definition or the docs for the $ operator, you'll see that its left argument is supposed to be a function.

     right argument
       |
       v
  0 $ (...)
  ^
  |
 left argument

But PureScript sees a 0 (of type Int) instead of a function.

And so it rightly complains: cannot match type Int with type (function type)

What I'm guessing you actually meant is to (1) try to convert the Number to an Int with fromNumber, and (2) if that fails, substitute zero with fromMaybe.

If you look at the definition of fromMaybe, you'll see that it has two arguments: first argument is the default value and second argument is the Maybe value. So all you need to do is just call fromMaybe with those two arguments:

                          second argument
                                 |
                         ------------------
                        /                  \
                        v                   v
myInteger = fromMaybe 0 (fromNumber myNumber)
                      ^
                      |
                  first argument

I really don't understand how you could possibly get the idea of writing 0 $ (...), so I cannot address that particular misunderstanding, sorry.

"I really don't understand how you could possibly get the idea of writing 0 $ (...), so I cannot address that particular misunderstanding, sorry." I tried it because it looks like JavaScript LOL. In JavaScript extra parentheses generally don't change the way the code works - so I've got a lot to learn about PS. Good answer tho! Got it working thanks — Code Whisperer, Apr 06 '22 at 16:30
This answer has prompted another question, which can be found here: https://stackoverflow.com/questions/71770525/purescript-frommaybe-doesnt-not-return-the-maybe-value — Code Whisperer, Apr 06 '22 at 16:36
In JavaScript extra parentheses can _absolutely_ change the meaning. E.g. `f(2, 3)` vs. `f(2 (, 3))` or `(f(2,) 3)`. Or `a[42]` vs. `(a[)42]`. Or `x.y` vs. `x.(y)` — Fyodor Soikin, Apr 06 '22 at 16:55

PureScript - Integer from Number using fromMaybe

1 Answers1