Introduce HuddleM #40
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This is an attempt at addressing #35. We provide a monad for specifying Huddle definitions, which will collect them in definition order. This allows a fairly convenient way to put together a specification, at the cost of it being trickier to re-use definitions elsewhere, since they need to be returned from the monad.
Soupstraw
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Nov 13, 2024
example/Monad.hs
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| setRootRules [transaction] | ||
| pure () |
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I wonder if it would make sense to return all of the roots instead of setting them via setRootRules
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The reason I didn't do this is that it might be needed to return specific things to allow them to be re-used.
By using an ordered map we allow a lot of information for sorting the entries arising from Huddle, and in particular for _merging_ such entries, where we want to override some definitions.
We add some tools to allow extending Huddle specifications using the semigroup instance established: - We allow one to reference a HuddleItem (which is a rule, a group, or a generic def) as a Type0. - We then allow one to reference (by name) a HuddleItem from an existing spec. These two combined allow us to (somewhat) "extend" a specification in a nice manner - we can reference items from the previous spec by their name, and then selectively override things using the semigroup instance (but respecting the original ordering). There are two "disappointments" involved in this, however: - Since the items from a previous spec are referenced by name, we lose the type-safety provided by the Haskell compiler. It's quite possible to refer to an item that doesn't exist, and you won't find out until runtime. - The whole thing falls apart for generic rules. When calling a generic rule in the usual way, we do two things: - Apply the actual argument and turn it into a 'GRuleCall' which we return to the call site. - Discard the argument, create an appropriate number of fresh names and insert this into Huddle as a 'GRuleDef'. This crucially ignores any actual arguments, which is why we can pass an error as a fake argument in the Includable instance for HuddleM and have it all work. Unfortunately, what we cannot do is go from the 'GRuleDef' and extract from it the fact that, on the Haskell side, this is a function (with an unknown number of parameters). Which is very annoying. I have some ideas about resolving this second issue in a slightly-less-horrible way; they will follow in a subsequent commit.
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This is an attempt at addressing #35.
We provide a monad for specifying Huddle definitions, which will collect them in definition order. This allows a fairly convenient way to put together a specification, at the cost of it being trickier to re-use definitions elsewhere, since they need to be returned from the monad.