[Haskell-cafe] Read Instance for UArray won't port to linux
simonpj at microsoft.com
Wed Mar 14 12:26:49 EDT 2007
Ah, a fine idea. I'll do that anyway; maybe others will have even better ideas, but that's a good start
| -----Original Message-----
| From: haskell-cafe-bounces at haskell.org [mailto:haskell-cafe-bounces at haskell.org] On Behalf Of Björn
| Sent: 14 March 2007 14:28
| To: Simon Peyton-Jones
| Cc: haskell-cafe at haskell.org
| Subject: Re: [Haskell-cafe] Read Instance for UArray won't port to linux
| Simon Peyton-Jones wrote:
| > | 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
| > | this.
| > 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
| > I can see why you read it that way. Now you understand, can you give me a better error message?
| I'm not the original poster, but what about just changing it to "The
| instance type must be of the form (T a1 ... an) where T is not a
| synonym, and a1 ... an are distinct type variables)"?
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