:set +m

let's define a function called f(x) = x + 1 in multiple lines mode

>:set +m
>let 
>f::Int->Int
>f x = x + 1 
>       -- the newline will end defintion of function f
>f 1
2

Apparently, there is some bug/issue in GHCi 7.10.3 for multiple line mode, e.g.
:{
ff::Int -> Int
ff x = x + 1
:}
ff 1, parse error on input ‘=’

GHCi 7.10.3 does not recognize 'prompt2' any more.
GHCi 8.4.3 use prompt-cont (stand for: prompt continuation)

Download the Haskell Framework (GHC and GHCi 8.4.3) and install it, 
above error has not gone away
The problem is the last line need to be EMPTY line. 
:{
ff::Int -> Int
ff x = x + 1
        <-- empty line here
:}