CODEWITHSUNDEEP
haskellscopeghc
marsella
marsella

Reputation: 123

Haskell Import error: Not in scope

I have written this code: 

import GHC.Float                                                                 

next :: GHC.Float -> GHC.Float-> GHC.Float                                         
next n x = (x + n / x) / 2

And I am getting the following error:

numerical.hs:3:9:
    Not in scope: type constructor or class `GHC.Float'

numerical.hs:3:22:
    Not in scope: type constructor or class `GHC.Float'

numerical.hs:3:34:
    Not in scope: type constructor or class `GHC.Float'

The module imports without any problem, so I'm not sure if I'm referring to it with the wrong name or if the standard Float module is the same as the IEEE GHC.Float one and there's no need to explicitly import it.

I tried doing an import GHC.Float as Fl with no success--got the same type error on Fl.

I'm just starting Haskell (obviously), so any help is appreciated!

Upvotes: 5

Views: 4240

Answers (1)

bheklilr
bheklilr

Reputation: 54058

You don't have to import GHC.Float manually, you can just write Float, like so

next :: Float -> Float -> Float
next n x = (x + n / x) / 2

GHC implicitly imports a module called Prelude in every source file you have. Prelude includes a lot of handy types, functions, and other things that are used as the "built-ins" of the language. Types like Int, Float, Maybe, IO, and functions like head, +, /, and more.

You can test to see if a floating point number is an IEEE floating point with the function isIEEE from the GHC.Float module:

import GHC.Float

main = do
    putStr "1.0 is an IEEE floating point: "
    print $ isIEEE (1.0 :: Float)

If you run this, it will print True

I should have also mentioned that the reason why your code didn't compile earlier is because when you import a module with just import, everything from it comes into scope. You can force it to be qualified by using import qualified, here's a few examples:

import GHC.Float -- Everything now in scope
import qualified Data.Maybe -- Have to use full name
import qualified Data.List as L -- aliased to L

main = do
    -- Don't have to type GHC.Float.isIEEE
    print $ isIEEE (1.0 :: Float)
    -- Have to use full name
    print $ Data.Maybe.isJust $ Nothing
    -- Uses aliased name
    print $ L.sort [1, 4, 2, 5, 3]

Upvotes: 5

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