Why is `2 + x = 7` valid Haskell?
When I try to compile
main = putStrLn $ show x where
2 + x = 7
GHC complains
error: Variable not in scope: x
|
1 | main = putStrLn $ show x
| ^
So it seems that 2 + x = 7 is by itself syntactically valid, although it doesn't actually define x . But why is it so?
It is valid because it defines + instead.
main = putStrLn (3 + 4)
where -- silly redefinition of `+` follows
0 + y = y
x + y = x * ((x-1) + y)
Above, the Prelude (+) function is shadowed by a local binding. The result will be 24 , not 7 , consequently.
Turning on warnings should point out the dangerous shadowing.
<interactive>:11:6: warning: [-Wname-shadowing]
This binding for ‘+’ shadows the existing binding
You're defining a local function called + .
2 + x = 7 is equivalent to (+) 2 x = 7 , which is equivalent to
(+) y x | y == 2 = 7
x is an (unused) parameter, and the function is only defined if the first argument is 2 . That's not very useful, but it explains why x is not visible outside.