Haskell error: parse error on input '='

I created a new quick.hs file in the ghci.exe directory. And the content is

quicksort::(Ord a)=>[a]->[a]  
quicksort []=[]  
quicksort (x:xs)=  
    let smaller = [a |a<-xs,a<=x]  
        larger = [a |a<-xs,a>x]  
    in  quicksort smaller ++ [x] ++ quicksort larger 

When I issue :l quick in the ghci command lline, the output is

Prelude> :l quick
[1 of 1] Compiling Main             ( quick.hs, interpreted )

quick.hs:5:17: error:
    parse error on input ‘=’
    Perhaps you need a 'let' in a 'do' block?
    e.g. 'let x = 5' instead of 'x = 5'
Failed, modules loaded: none.

I have concured this kind of problems many times. What's wrong on earth?