On Tue, May 31, 2011 at 20:48, Jake Penton
Ok, so far so good. But the following example seems quite astounding to me: ghci> (5 * 10 + 2)::SymbolicManip Int Arith Plus (Arith Mul (Number 5) (Number 10)) (Number 2) The book says to notice that haskell "converted" 5 * 10 + 2 into a SymbolicManip. Indeed! My understanding breaks down at this point. I suspect that it may be my weak grasp of the implications of lazy evaluation, and typeclasses generally. My
It's not lazy evaluation; it's the combination of types and the automatic translation of literal values to calls to fromLiteral per the Haskell language definition. Since in this case, the type produced by fromLiteral must be a SymbolicManip Int for the entire expression to work out to a SymbolicManip Int, each literal becomes (Number whatever) and then the operators are taken from SymbolicManip to preserve the type.