On 19 Jul 2009, at 21:18, Yitzchak Gale wrote:
Hi Phil,
I've concocted a very simple example to illustrate this (below) - but it doesn't compile because ghc complains that my type is ambiguous arising from my use of 'fromSeq'.
Notice that you have given two completely separate sets of instructions of what to do depending on whether Int or Double is selected. You have not given any indication of how to choose between them, even at runtime. Of course, the compiler doesn't care that your string constants "Int" and "Double" happen also to be the names of types if unquoted.
I see now. I'm passing fromSeq a SeqType, but it has no way of knowing if I want to process it as an Int or a Double. The only thing which is polymorphic is nextSeq as it must handle the underlying state of Int and Double. Your result function handles the general case and the typeclass instances deal with the specialization depending on the state's type. The printResult function takes in a SeqType and then "parses" (for want of a better word) out the underlying type of Int or Double. It then calls results against the Int or Double which in turn will invoke the correct version of nextSeq. Thank you very much for explaining this! Phil.
import Control.Monad.State -- Why Strict? Haskell is lazy by default.
Ahh, no reason for the Strict - in the large program I'm righting it is required because otherwise I end up with almighty thunks. But here it serves no purpose.