[Haskell] Re: Implicit type of numeric constants

Robert Stroud R.J.Stroud at ncl.ac.uk
Thu Sep 21 10:34:51 EDT 2006

On 20 Sep 2006, at 22:21, Ashley Yakeley wrote:

> Arie Peterson wrote:
>> You absolutely right about this defaulting breaking referential  
>> transparency.
> Do you know if it can be switched off in GHC? I know one can switch  
> on warnings when it happens, but I don't think that's the same thing.

You can use an empty default declaration to switch off default types  
for a particular module, but I don't think you can do this for all  
modules, or at least not in standard Haskell - section 4.3.4 of the  
language manual says:

"Only one default declaration is permitted per module, and its effect  
is limited to that module. If no default declaration is given in a  
module then it assumed to be:

   default (Integer, Double)

The empty default declaration, default (), turns off all defaults in  
a module."

Switching off default types may introduce error messages about top- 
level declarations with unbound type variables - for example

     Ambiguous type variable `t' in the constraint:
       `Num t' arising from the literal `2' at /tmp/test.hs:3:4
     Possible cause: the monomorphism restriction applied to the  
       k :: t (bound at /tmp/test.hs:3:0)
     Probable fix: give these definition(s) an explicit type signature
                   or use -fno-monomorphism-restriction

Turning off the monomorphism restriction makes the errors go away,  
but adding an explicit polymorphic type signature is probably better.

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