Re: [Haskell-beginners] Question on Real World Haskell ch.13 SymbolicManip

Thanks. See below. On 2011-05-31, at 9:14 PM, Brandon Allbery wrote:

On Tue, May 31, 2011 at 20:48, Jake Penton wrote:

...

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

I don't see fromLiteral in the language definition. Is it fromInteger that I should look at, maybe?

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.