Why does my code in Haskell work on command line but not in a file

Question

I have the following code that works well on command line:

ghci> [let square x = x * x in (square 5, square 3, square 2)]
[(25,9,4)]

but when I have it in a file and compile it, it doesn't work and I get an error:

[1 of 1] Compiling Main             ( baby.hs, interpreted )

baby.hs:62:1:
    Parse error: naked expression at top level
    Perhaps you intended to use TemplateHaskell
Failed, modules loaded: none.

Answer 1

The REPL, GHCi, accepts Haskell expressions. Unlike Python, a Haskell module must be composed of declarations.

For example, an expression could be 1+1 but even from a human perspective that makes no sense as a top-level of a source file - there is no variable declared and no operation performed. At the top level you can declare a value, such as:

val = [let square x = x * x in (square 5, square 3, square 2)]

Or in the REPL you can do the same but with let:

let val = [let square x = x * x in (square 5, square 3, square 2)]