I have the following simple program for which Hat does not seem to produce compilable code.
main = putStr (show $ 4 ^ 2)
Any suggestions?
Yes. You came across a restriction of Hat which, as I just notice, is unfortunately not documented. This example needs type defaulting to work. You can see in Hugs Prelude> :t putStr (show $ 4 ^ 2) putStr (show $ 4 ^ 2) :: (Integral a, Num b) => IO () "a" and "b" are the types of "4" and "2", and the defaulting mechanism of Haskell sets them to "Integer". In the Hat-transformed code defaulting does not apply, because the transformed code uses different classes "Integeral", "Num" etc, but by definition defaulting only applies to a set number of standard numeric classes. The simple solution is to add type annotations: main = putStr (show $ (4::Integer) ^ (2::Integer)) Don't worry, in practice this situation occures very rarely. (Only twice in the Haskell standard libraries.) However, an "ambiguous type variable" error message is a clear indicator of this problem. To locate in your module where you have to place a type annotation, just add "default ()" to the module and compile it normally. Olaf -- OLAF CHITIL, Dept. of Computer Science, The University of York, York YO10 5DD, UK. URL: http://www.cs.york.ac.uk/~olaf/ Tel: +44 1904 434756; Fax: +44 1904 432767