Parse error on input '=' something wrong with my Haskell

Question

This is a simple function:

len [] = 0    
len (x:xs) = 1 + len xs

and I have loaded it in to GHCi using :l, but I aways got this error parse error on input =.

I run this in another computer, then it's OK. My computer is a Mac. Is there something wrong with my Haskell?

It works in my computer, with Ubuntu 12.04 and GHC 7.4.1. I copy pasted your code snippet and it works just fine. — Andres F.
– Andres F., Commented Apr 20, 2013 at 16:50

Mark Reed · Accepted Answer · 2013-04-20 02:37:21Z

6

You need a newline between the two patterns for the len function. Then it works fine:

$ cat len.hs
len [] = 0
len (x:xs) = 1 + len xs
$ ghci
GHCi, version 7.4.1: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude> :l len.hs
[1 of 1] Compiling Main             ( len.hs, interpreted )
Ok, modules loaded: Main.
*Main> len []
0
*Main> len [1]
1
*Main> len [1,2,3]
3
*Main>

Since you mentioned that it's a Mac, perhaps you have a newline-convention incompatibility. Make sure that your text editor and GHCi agree about what constitutes a newline on your platform.

edited Apr 20, 2013 at 2:37

answered Apr 20, 2013 at 2:30

Mark Reed

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2 Comments

user2299776 Over a year ago

Oh, there is a new line in my file, not working. Wrong format on the website.

Mark Reed Over a year ago

As I said, make sure it's the sort of newline that GHC is expecting, because the error you are getting is exactly the error you would get if the newline were missing. Is it a carriage return, linefeed, or both?

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