[Hat] Program Compiles With GHC 6.6 but not Hat

[Olaf Chitil]

This is caused by a limitation of Hat: Hat cannot handle defaulting of 
numeric classes.

Your Haskell program makes use of defaulting: The main function could 
work with numbers of either type Int or Integer. The Haskell defaulting 
rule says that in such a case it should choose Integer.

To help Hat add a type annotation, e.g.

main = do { print (takeodds [(1::Integer) ..7]); }

To get a better error message, you could compile your original program 
without -hat but with the additonal line

default()

which turns defaulting off and thus will yield an error message about an 
ambiguous type variable referring to your original program.

In real programs we have hardly ever come across uses of defaulting.

Ciao,
Olaf


Aditya Siram wrote:

> The following program compiles and runs on GHC 6.6 but fails to 
> compile with the '-hat' option to hmake. The error message is:
> **Begin Output**
> hat-trans   HatTest.hs
> Wrote Hat/HatTest.hs
> ghc-6.6      -c -package hat -o Hat/HatTest.o Hat/HatTest.hs
>
> Hat/HatTest.hs:40:50:
>    Ambiguous type variable `a' in the constraint:
>      `Integral a'
>        arising from use of `htakeodds' at Hat/HatTest.hs:40:50-58
>    Probable fix: add a type signature that fixes these type variable(s)
> **End Output**
>
> The program source is:
>
> import Prelude
>
> takeodds xs = [ x | x <- xs, (mod x 2) ==0]
>
> main = do { print (takeodds [1 ..7]); }
>
>
> Thanks...
> Deech
>
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