How to fix "parse error on input" in haskell?

Question

Prelude Data.Set> :load hello
[1 of 1] Compiling Main             ( hello.hs, interpreted )

hello.hs:11:11: parse error on input `<-'
Failed, modules loaded: none.
Prelude Data.Set> h <- IO.openFile "testtext" IO.ReadMode
Prelude Data.Set> 

The same line [h <- IO.openFile "testtext" IO.ReadMode] inside hello.hs throws the error. How do i fix this? What am I doing wrong?

[EDIT] Source and output: http://pastebin.com/KvEvggQK

Solution

You can only use <- inside a do-block¹ (which you're implicitly in in GHCI, but not in Haskell files).

In a Haskell file, you're only allowed to write bindings using =.

What you could do is put the following in the Haskell file:

myHandle = do h <- IO.openFile "testtext" IO.ReadMode
              return h

Though if you think about that for a bit, this is just the same as:

myHandle = IO.openFile "testtext" IO.ReadMode

However this way myHandle is still wrapped in IO and you'll need <- (or >>=) in ghci to unwrap it.

You can't write a Haskell file in such a way that just loading the file, will open testtext and give you the file handle.

---

¹ Or a list comprehension, but there the right operand of <- needs to be a list, so that has nothing to do with your situation.

Comments

[sepp2k]

This is completely wrong. You can't use IOs as the right operand to <- in a list comprehension.

[Oswald]

My answer is at most partially wrong: [h <- IO.openFile "Copy Bin.txt" IO.ReadMode] produces a parse error, [h | h <- IO.openFile "Copy Bin.txt" IO.ReadMode] does not produce a parse error. Depending on the definition of IO.openFile it might not produce any error at all. Granted, it produces type error when the most common definition is used.