I trust that "x = 10" above does NOT t mean the same as "let x = 10".
If it did, then it wouldn't mean what it does in Haskell.
Then code tried successfully in GHCI could fail in GHC.

Compare this GHCI session
        > let x = 10
        > :t x
        x :: Num a => a
        > x + 0.0
        10.0
to this, in which x = 10 has its Haskell meaning
        > :! cat test.hs
        x = 10
        > :load test.hs
        > :t x
        x :: Integer
        > x + 0.0
        No instance for (Fractional Integer) arising from the literal ‘0.0’.