I have the following sample of code:
{-# LANGUAGE ScopedTypeVariables #-}
main = do
putStrLn "Please input a number a: "
a :: Int <- readLn
print a
putStrLn "Please input a number b: "
b :: Int <- readLn
print b
putStrLn ("a+b+b^2:" ++ (show $ a+b+c))
where c = b^2
For some reason I cannot use variable b
in a where
clause, the error I get is the following:
Main3.hs:13:15: error: Variable not in scope: b
|
13 | where c = b^2
| ^
Any ideas how to make b
available in the where
clause?
Use let
instead of where
:
{-# LANGUAGE ScopedTypeVariables #-}
main = do
putStrLn "Please input a number a: "
a :: Int <- readLn
print a
putStrLn "Please input a number b: "
b :: Int <- readLn
print b
let c = b^2
putStrLn ("a+b+b^2:" ++ (show $ a+b+c))
The reason for the problem is that variables in the where
clause are in scope for all of main
, but b
isn't in scope until after b :: Int <- readLn
. In general, where
clauses can't reference variables bound inside of a do
block (or anywhere to the right of the =
, for that matter: e.g., f x = y*2 where y = x+1
is fine but f = \x -> y*2 where y = x+1
is not).
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