Projects Attribute Grammar Future Projects
HUT
Unordered List
- Derive data type structure from Haskell code (when data types are sufficiently simple; use package haskell-src-exts)
- Type-attributes (for polymorphism in semantics; see my thesis, chapt dep-type AGs)
- Namespaces for attributes (to prevent name clashes: mainly useful for local attributes)
- Extensions for symbol tables using def/use policies, potentially in combination with ordering info.
- New ordering algorithm where you can (optionally) make visits explicit, avoid scheduling rules over certain visits, etc. (see my thesis, iterative AGs; only factor out the ordering algorithm)
- Have different sets of inh/syn attrs per nonterminal (see my thesis, chapt dep-type AGs; needs GADTs). Mainly useful in combination with HO-attrs.
Polymorphic Attributes
Currently, UUAGC does not allow polymorphism in the types of attributes. For code reuse purposes, it is sometimes convenient to parametrize the types of attributes. We achieve this goal via type-level attributes. In the following example, we introduce two type-level attributes,a and b respectively, and refer to these in the types of attributes:
DATA Apply | App
ATTR Apply [ fun : {@a -> @b} arg : {@a} a :: {*} b : {*} | | res : {@b} ]
SEM Apply | App lhs.res = @lhs.fun @lhs.arg
Inherited type-level attributes correspond to universally quantified types. Synthesized type-level attributes are existentially quantified. Note: to allow synthesized type-level attributes, it may be necessary to have an ordered attribute grammar, such that these existentials can be opened via case-statements. Also, it is necessary to wrap the semantic-type into a data-type.
The above code is translated to:
data T_Apply _a _b where T_Apply :: (_a -> _b) -> _a -> _b -> T_Apply _a _b sem_Apply_App :: T_Apply _a _b sem_Apply_App = \lhsIfun lhsIarg -> let lhsSres = lhsIfun lhsIarg in lhsSresTo require a dictionary on a polymorhpic type, we represent it explicitly using a data-type and pass it as inherited attribute:
data DictShow a where
DictShow :: Show a => DictShow a
ATTR Apply [ d : {DictShow @a} | | ]
SEM Apply | App
lhs.res = case @lhs.d of
DictShow -> trace (show @lhs.arg) (@lhs.fun @lhs.arg)
In general, we generate the following type for a nonterminal with inherited type-attributes inhT1, inhT2 (respectively synT3 and synT4), and
inherited value-attributes inhV5, inhV6 (respectively synV7, synV8) to:
data T_Nt inhT1 inhT2 where T_Nt :: forall synT3 synT4 . ( inhV5 -> inhV6 -> (synV7, synV8) ) -> T_Nt inhT1 inhT2This requires an additional pattern match on the semantic result of a child, analogous to what the
--newtypes extension already does.