Type defaulting

Stil not clear how this works:

• module Main where
   
  main :: IO ()
  main = do
      print $ negate 4

Output:

• -4

…or this:

• module Main where
   
  main :: IO ()
  main = do
      print $ sin 3

Output:

• 0.1411200080598672

Despite, lentgthy discussions here: https://www.reddit.com/r/haskell/comments/1r3w3w/implicit_type_conversion_for_value_constants/

And despite this: https://www.haskell.org/onlinereport/decls.html#sect4.3.4

And despite this: https://kseo.github.io/posts/2017-01-04-type-defaulting-in-haskell.html

…which states: “Haskell default rule can be summarized as:”

• default Num Integer
  default Real Integer
  default Enum Integer
  default Integral Integer
  default Fractional Double
  default RealFrac Double
  default Floating Double
  default RealFloat Double

Latest reserach in GHC.Num shows:

• module GHC.Num (module GHC.Num, module GHC.Integer, module GHC.Natural) where
   
  #include "MachDeps.h"
   
  import GHC.Base
  import GHC.Integer
  import GHC.Natural
   
  infixl 7  *
  infixl 6  +, -
   
  default ()              -- Double isn't available yet,
                          -- and we shouldn't be using defaults anyway

Even more puzzling, this here does not compile:

• module Main where
   
  default ()
   
  main :: IO ()
  main = do
      print $ sin 3

Compiler error:

• app\Main.hs:7:5: error:
      * Ambiguous type variable `a0' arising from a use of `print'
        prevents the constraint `(Show a0)' from being solved.
        Probable fix: use a type annotation to specify what `a0' should be.
        These potential instances exist:
          instance Show Ordering -- Defined in `GHC.Show'
          instance Show Integer -- Defined in `GHC.Show'
          instance Show a => Show (Maybe a) -- Defined in `GHC.Show'
          ...plus 22 others
          ...plus 28 instances involving out-of-scope types
          (use -fprint-potential-instances to see them all)
      * In a stmt of a 'do' block: print $ sin 3
        In the expression: do print $ sin 3
        In an equation for `main': main = do print $ sin 3
    |
  7 |     print $ sin 3
    |     ^^^^^^^^^^^^^

It will work again, like this:

• module Main where
   
  default (Float)
   
  main :: IO ()
  main = do
      print $ sin 3

Output:

• 0.14112