Maurício comments the remark of Ketil Malde
Note that Haskell doesn't automatically convert arguments for you -- this is a feature.
When I type this:
***** import Complex; a = 3 :+ 4; *****
and load it into ghci, "a + 4" gives me "7.0 :+ 4.0", although "a + (4::Float)" gives me that error again. Why Haskell converts "4" to Complex but not a Float?
You should read thoroughly the Haskell documentation. You will learn that Haskell casts EXPLICIT NUMERICAL CONSTANTS, but not variables. Actually this is not a "conversion", but an overloading of numerical constants. The lexical entity "4" behaves as "fromInteger 4", and the type checker uses the fromInteger appropriate to the context. Floating, Complex, etc. With 4.0 it will work also (overloaded fromRational). Jerzy Karczmarczuk