Question

I’m reading the guide in learnyouahaskell.com . One sentence mentioned “where” can be shared globally, but no example was given, so where can I find some details, please?

“where bindings aren’t shared across function bodies of different patterns. If you want several patterns of one function to access some shared name, you have to define it globally.”

Answer 1

From Chapter 4: Syntax in Functions:

where bindings aren’t shared across function bodies of different patterns. If you want several patterns of one function to access some shared name, you have to define it globally.

Here is an illustration:

f (Left x) = double x
f (Right x) = double x
    where
    double x = 2 * x

The function f has a body for each of the patterns (Left x) and (Right x). The binding of double isn’t shared across the function bodies, and therefore this code isn’t valid Haskell.

If we want to access double from both of the function bodies, we have to move it out of the where clause:

double x = 2 * x

f (Left x) = double x
f (Right x) = double x

That’s all that the quoted paragraph means.