88 lines
3.3 KiB
Plaintext
88 lines
3.3 KiB
Plaintext
2015-05-28
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Problem: how to represent \forall types
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1) (Racket) functions
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- this gets the most linguistic reuse (is this true?)
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- but this does not allow equality comparisons
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- unless perhaps compare two foralls for equality by applying to the same tvar
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- but these type vars would be unbound so it still wouldnt work without
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adding a new special case
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2) syntax
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- easier to compare
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- but still need to manually implement alpha equality
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- would still require a special case for comparing the bodies, which have
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unbound typevars
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Problem: begin in lambda body gets spliced
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- results in combined syntax properties, eg types
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Solution:
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- wrap lambda body with #%expression to indicate expression, ie non-splicing,
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begin
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2015-05-29
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Notes: locally-nameless representation of lambdas (and other binding terms)
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- syntactically distinguishes bound names vs free vars
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- combination of debruijn (nameless) for bound vars and names
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- simplifies implementation of capture avoiding substitution
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- I already get around by using Racket's identifiers and free-identifier=
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to easily implement capture-avoiding subst
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- debruijn indices for boundvars avoids having to convert to canonical form
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to compare for alpha-equal
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- using names for free vars avoids "shifting" of indices when adding or
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removing binders, ie free vars dont rely on context
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- two main operations:
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- open: converts some bound vars to free vars
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- eg subst into lambda body
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- conversion and subst can be done in one pass?
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- close: converts some free vars to bound vars
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- eg wrapping a term in a lambda
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- similar to subst
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- both operations involve traversing the term
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- but can do straight-subst (instead of renaming subst) because
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shadowing is not possible
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- multiple binders are more complicated
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- require both depth and offset index
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Previous: -----------------
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macro system requirements:
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- depth-first expansion, i.e., localexpand, and stop-lists
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- language form hooks, e.g., #%app, etc
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- literal types, e.g. integer syntax class, ie compile time literal type tag
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- identifiers and free-identifier=?
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- syntax-parse or other pattern matching
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Type constructors must be prefix (and not infix) and must be functions
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- because in order to support type aliases:
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- types must be expanded,
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- and having a macro identifier (ie, an alias) in the function position
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makes the expander error (constructor is ok bc it is run time identifier)
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Type expansion problem: what to do about #%app?
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1) use the #%app in scope:
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- may do type checking and error bc types dont have types
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2) use the racket #%app:
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- may work but how to do this without ruining context of other
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identifiers (ie types)
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Solution: do #1, but
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1) stop at the #%app
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2) manually drop it and continue expanding rest
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Types must be identifiers, but not macros
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- cannot be macros if we want to use expansion for type aliases
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- because then what does a base type like Int expand to?
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- if we define Int as a runtime identifier, then expansion will stop at Int
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debugging notes -------------
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- "datum" error:
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?: literal data is not allowed;
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no #%datum syntax transformer is bound in: #f
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- likely indicates use of wrong version of some overloaded form
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- eg, using stlc:lambda instead of racket's lambda
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- vague "bad syntax" error
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- means a syntax-parse #:when or #:with matching failed
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- ideally would have better err msg at that spot
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