Experiments with modularisation of lambda calculus interpreter using datatypes à la carte.

• Expr.hs – bare lambda calculus
• Prim/Base.hs – base primitives and values (unit, pair, double)
• Prim/Kappa.hs – primitives and values for kappa calculus residual program of a two-level language
• Prim/Record.hs – support of records (create, extend, select)
• Prim/Variant.hs – support of variants (create, eliminate)
• Prim/IO.hs – support of IO (read line from stdin, write line to stdout)
• Prim/Exception.hs – support of exceptions (raise, catch)
• Prim/Link.hs – support of dynamic loading of expressions from other files, typechecking them and running in the same runtime context as the parent program runs

• [X] Modularised evaluation, typechecking, side-effecting, and pretty-printing aspects of sublanguages
• [X] Add variant construction and elimination (case expressions)
• [X] Make exceptions use variants
• [X] Type system with higher-rank polymorphism and bidirectional type-checker
• [ ] Add Koka-style extensible effects to bidirectional type-checker
• [ ] Add isorecursive types (μ-types)
• [ ] Add paramorphisms (== primitive recursion) for elimination of μ-types
• [ ] Add type variable constraints; needed for overloaded literals and operations on numbers
• [ ] Add lists and on-the-fly μ-type/paramorphisms machinery for elimination
• [ ] Use subsumption checking and coercion for dynamically loaded expressions
• [ ] Add vector type and vector sublanguage (and the Nat kind?)
• [ ] Patterns and pattern-matching