I was writing a function something similar to this:
f x = let
x = ...
in
e
Due to scoping rules in Haskell any use of x in e will resolve to the definition of x in the let construct.
Why is such a thing allowed in Haskell? Shouldn't the compiler reject such a program telling we cannot bind a value that has the same name as argument of the function.
(This example may be simplistic, but in real world context where variables have semantic meaning associated with them it is easy to make such mistake)
You can enable warnings for this type of name shadowing with the compiler flag
This option causes a warning to be emitted whenever an inner-scope value has the same name as an outer-scope value, i.e. the inner value shadows the outer one. This can catch typographical errors that turn into hard-to-find bugs, e.g., in the inadvertent capture of what would be a recursive call in f = ... let f = id in ... f ....
However, it is more common to compile with -Wall, which includes a lot of other warnings that will help you avoid bad practices.