Haskell error - 'parse error on input `->'

Question

I am trying to declare a function in Haskell GHCi as

fact :: Int -> Int

But I am getting this error - error: parse error on input `->'

I do not understand why this is happening. Can anyone please explain it to me? Thanks.

I get "error: Variable not in scope: fact :: Int -> Int" (AFAIK, you can't "forward-declare" a type in ghci). Which version are you using, and is that all your input? — molbdnilo
@Megha That would work with the compiler, but with GHCi the syntax has to be subtly different. This however works with GHCi v8.2.2: let { fact :: Int -> Int ; fact 0 = 1 ; fact n = n * fact (n-1) } — jpmarinier

leftaroundabout leftaroundabout · Accepted Answer · 2019-11-08T10:10:51

First off, it looks like you're using a pretty old version of GHC. In newer versions, the GHCi syntax has been relaxed a bit.

But still: what you type in GHCi does not have the same rules as what you write in a Haskell source file. Specifically, the GHCi prompt is essentially an IO monad chain evaluator, the reason being that you can write stuff like

Prelude> putStrLn "Hello"
Hello

or

Prelude> readFile "test.txt" 
"fubar\nbaz"

and actually have it execute right there. By contrast, in a Haskell source file, you only declare bindings, and these can then be invoked in the main action or a GHCi session.

But in this case, you want to declare a binding within GHCi itself. You can do that too, but it's a bit awkward, basically you need to start with let and then squeeze everything in a single line:

Prelude> let fact :: Int -> Int; fact n = product [1..n]

Actually, newer GHCi version allow you to omit the let, and you can have multiple-line definitions by using a special bracket syntax:

Prelude> :{
Prelude| fact :: Int -> Int
Prelude| fact n = product [1..n]
Prelude| :}

but I would recommend against this. If you actually have some bigger definitions, better put them in a proper Haskell source and load that into GHCi.

Haskell error - 'parse error on input `->'

1 Answers