Mysterious type error with Glasgow Haskell

[Simon Peyton-Jones]

George

For a change, this is your fault!  You wrote:

    let
	minus :: Int -> Int -> Int
	minus = (-)

            toRValue :: Int -> Radio value
            toRValue -1 = NoRadio	-- (*)
            toRValue i = ...

Alas, the line marked (*) defines the infix operator (-).  So
in effect you have

    let
	(-) toRValue 1 = NoRadio

	minus = (-)

	toRValue i = ...

And then the result you get isn't surprising.  Solution: put
parens round your (-1).  

I don't see how to give a better error message given the
defn of Haskell as it is!  But it's a nice example.

Simon

| -----Original Message-----
| From: George Russell [mailto:ger at tzi.de] 
| Sent: 18 October 2001 15:55
| To: glasgow-haskell-bugs at haskell.org; Simon Peyton-Jones; Simon Marlow
| Subject: Mysterious type error with Glasgow Haskell
| 
| 
| Hello Glasgow hackers (cc'd to both Simons as the mailing 
| list will complain that this bug report is too big).
| 
| If you don't redefine the (-) operator, then
| 
|    minus :: Int -> Int -> Int
|    minus = (-)
| should work, shouldn't it?  Also it should be equivalent to
|    minus :: Int -> Int -> Int
|    minus a b = a + (-1) * b
| 
| The attached bzip2 file contains (1) a lot of .hi files; (2) 
| a file SimpleForm.hs. Compiling it with ghc5.02 on 
| Sparc/Solaris, with the following command line:
| 
| /usr/local/pub-bkb/ghc/ghc-latest/bin/ghc -c 
| toolkit/SimpleForm.hs -package concurrent -package data 
| -package net -package posix -package text -package util 
| -package lang 
| -i.:packer:resources:canvasitems:kernel:containers:menuitems:t
| oolkit:components:toplevel:widgets:devices:textitems:tix:/home
| /ger/uni/util:/home/ger/uni/events:/home/ger/uni/reactor:/home
| /ger/uni/server:/home/ger/uni/htk:/home/ger/uni/davinci:/home/
| ger/uni/graphs:/home/ger/uni/tools:/home/ger/uni/cvs:/home/ger
| /uni/types -fglasgow-exts -fallow-overlapping-instances 
| -fallow-undecidable-instances -cpp -ddump-hi-diffs -H25M 
| -recomp -fwarn-deprecations -Onot -DDEBUG -Onot -H30M -fasm 
| 
| gives an error:
| 
| toolkit/SimpleForm.hs:379:
|     Couldn't match `Int' against `Radio x'
|         Expected type: Int -> Int -> Int
|         Inferred type: Int -> Int -> Radio x
|     In the definition of `minus': (-)
| 
| However if you go to line 379, comment out the line "minus = 
| (-)" and decomment the next line to give "minus a b = a + 
| (-1) * b" the error message magically disappears and it goes 
| through the compiler.
| 
| Am I somehow mistaken, or is this a bug in GHC?
|