[Haskell-cafe] Read Instance for UArray won't port to linux
simonpj at microsoft.com
Wed Mar 14 05:21:29 EDT 2007
| The error in linux is:
| Illegal instance declaration for `Read (UArray Int Double)'
| (The instance type must be of form (T a b c)
| where T is not a synonym, and a,b,c are distinct type variables)
| In the instance declaration for `Read (UArray Int Double)'
| Why does it want three parameters for the instance type? I am baffled by
It's saying that in Haskell 98 you can have
instance Read (UArray a b)
(for type variables a, b) but not
instance Read (UArray Int Double)
(As Spencer says, use -fglasgow-exts to lift the H98 restriction.)
However, you mis-read the error message to say "the instance type must have three parameters", which isn't what I meant at all! I was trying to use an example of the general form, but conveyed the wrong idea.
I can see why you read it that way. Now you understand, can you give me a better error message?
More information about the Haskell-Cafe