# symantic

This is an experimental library for composing, parsing, typing, compiling, transforming and interpreting
a custom DSL (Domain-Specific Language).

# Features

Those custom DSL can express a subset of GHC's Haskell type system:

* *first class functions* (aka. *lambdas*),
* chosen *monomorphic types* (like `Bool` or `Maybe`),
* chosen *rank-1 polymorphic types* (like `(Maybe a)`),
* chosen *type class instances*,
* chosen *type family instances*,
* and chosen *type constraints*;

where "chosen X" means declared in Haskell
and selected when composing the DSL.

In particular, this library is currently not able to:

* do *type inferencing* for the argument of *lambdas*
  (they must all be explicitely annotated, aka. *Church-style*),
* do *pattern matching* (aka. *case*) (but *Church-encoding* functions are often enough),
* do *rank-N polymorphic types* (aka. *non-prenex forall*, like `(forall s. ST s a) -> a`).

And by itself, the DSL is only able to define new terms to be interpreted,
no new types, or other type-level structures.

# Warning

Please be aware that despite its using of powerful ideas from clever people,
this remains a FUND-LESS SINGLE-PERSON EXPERIMENTAL LIBRARY.
Meaning that it IS NOT heavily tested and documented,
DOES NOT have a strong commitment to preserving backward compatibility,
MAY FAIL to comply with the [PVP](http://www.haskell.org/haskellwiki/Package_versioning_policy),
and CAN die without notice.
You've been warned.

# Use cases

The main goal of this library is to enable the runtime interpretation of terms,
type-checked according to some types defined at composing-time (ie. GHC's compile-time).

Using a DSL enables to limit expressiveness in order to ease analysis.
Here the idea is that the more complex logic shall remain written in Haskell,
and then this library used to project an interface into a DSL
(using GHC's Haskell as a FFI (Foreign Function Interface)).
This in order to give runtime users the flexibility
to write programs not requiring a full-blown Haskell compiler,
yet enabling enough flexibility to let them express complex needs
with a reasonably advanced type-safe way
and a controlled environment of primitives.

## Typical use cases

* Enabling runtime users to enter some Haskell-like expressions
  (maybe with a more convenient syntax wrt. the domain problem)
  without using at runtime all the heavy machinery and ecosystem of GHC
  (eg. by using [hint](https://hackage.haskell.org/package/hint)),
  but still leaning on primitive functions coded in GHC's Haskell.
* Limiting those expressions to be built from well-controlled expressions only.
* Run some analyzes/optimizations on those well-controlled expressions.
* Report errors specific to the domain problem.

# Usage

Please learn how to use this library by reading example source files in `test/`
in [symantic-lib](https://hackage.haskell.org/package/symantic-lib)
in [Git repository](git://git.autogeree.net/symantic).

These `test` files use [megaparsec](https://hackage.haskell.org/package/megaparsec) as parser
(see `test/Testing/Megaparsec.hs`) and a default grammar somehow sticking to Haskell's,
but staying context-free (so in particular: insensitive to the indentation),
and supporting prefix and postfix operators.
This grammar — itself written as a symantic embedded DSL
with [symantic-grammar](https://hackage.haskell.org/package/symantic-grammar) —
can be reused to build other ones, is not bound to a specific parser,
and can produce its own EBNF rendition.

# Acknowledgements

This library would probably be much worse than it is
without the following seminal works:

* [Finally Tagless](http://okmij.org/ftp/tagless-final/) by Jacques Carette, Oleg Kiselyov, and Chung-chieh Shan.
* [Dependent Types in Haskell](http://cs.brynmawr.edu/~rae/papers/2016/thesis/eisenberg-thesis.pdf) by Richard A. Eisenberg.
* [A reflection on types](https://www.microsoft.com/en-us/research/wp-content/uploads/2016/08/dynamic.pdf) by Simon Peyton Jones, Stephanie Weirich, Richard A. Eisenberg and Dimitrios Vytiniotis.
* [Typeable](https://ghc.haskell.org/trac/ghc/wiki/Typeable) by Ben Gamari and others.

# Main ideas

* __Symantic DSL__.
  Terms are encoded in the [Tagless-Final](http://okmij.org/ftp/tagless-final/) way (aka. the *symantic* way)
  which leverages the *type class* system of Haskell — instead of using *data types* — to form an embedded DSL.
  More specifically, a *class* encodes the *syntax* of terms (eg. `Sym_Bool`)
  and its *class instances* on a dedicated type encodes their *semantics*
  (eg. `(Sym_Bool Eval)` interprets a term as a value of its type
  in the host language (`Bool` in Haskell here),
  or `(Sym_Bool View)` interprets a term as a textual rendition, etc.).
  
  DSL are then composed/extended by selecting those symantic *classes*
  (and in an embedded DSL those could even be automatically inferred,
  when `NoMonomorphismRestriction` is on).
  Otherwise, when using symantics for a non-embedded DSL
  — the whole point of this library — the *classes* composing the DSL
  are selected manually at GHC's compile-time,
  through the *type-level list* `ss` given to `readTerm`.
  
  Moreover, those symantic `term`s are parameterized by the type of the value they encode,
  in order to get the same type safety as with plain Haskell values.
  Hence the symantic *classes* have the higher kind: `((* -> *) -> Constraint)`
  instead of just `(* -> Constraint)`.
  
  Amongst those symantics, `Sym_Lambda` introduces *lambda abstractions* by an higher-order approach,
  meaning that they directly reuse GHC's internal machinery
  to abstract or instantiate variables,
  which I think is by far the most efficient and simplest way of doing it
  (no (generalized or not) DeBruijn encoding
  like in [bound](https://hackage.haskell.org/package/bound)'s `Monad`s).

* __Singleton for any type__.
  To typecheck terms using a `(Type src vs t)` which acts as a *singleton type*
  for any Haskell *type index* `t` of any kind,
  which is made possible with the dependant Haskell extensions:
  especially `TypeFamilies`, `GADTs` and `TypeInType`.

* __Type constants using `Typeable`__.
  *Type constant* could be introduced by indexing them amongst a *type-level list*,
  but since they are *monomorphic types*, using `Typeable` simplifies
  the machinery, and is likely more space/time efficient, including at GHC-compile-time.

* __Type variables using a type-level list__.
  Handling *type variables* is done by indexing them amongst a `vs` *type-level list*,
  where each *type variable* is wrapped inside a `Proxy` to handle different kinds.
  Performing a substitution (in `substVar`) preserves the *type index* `t`,
  which is key for preserving any associated `Term`.
  Unifying *type variables* is done with `unsafeCoerce` (in `unifyType`),
  which I think is necessary and likely safe.

# Main extensions

* `AllowAmbiguousTypes` for avoiding a lot of uses of `Proxy`.
* `ConstraintKinds` for *type lists* to contain `Constraint`s,
  or reifying any `Constraint` as an explicit dictionary `Dict`,
  or defining *type synonym* of *type classes*,
  or merging *type constraints*.
* `DataKinds` for type-level data structures (eg. *type-level lists*).
* `DefaultSignatures` for providing identity transformations of terms,
  and thus avoid boilerplate code when a transformation
  does not need to alter all semantics.
  Almost as explained in [Reducing boilerplate in finally tagless style](https://ro-che.info/articles/2016-02-03-finally-tagless-boilerplate).
* `GADTs` for knowing types by pattern-matching terms,
  or building terms by using type classes.
* `PolyKinds` for avoiding a lot of uses of `Proxy`.
* `Rank2Types` or `ExistentialQuantification` for parsing `GADT`s.
* `TypeApplications` for having a more concise syntax
  to build `Type` (eg. `tyConst @Bool`).
* `TypeFamilies` for type-level programming.
* `TypeInType` (introduced in GHC 8.0.1)
  for `Type` to also bind a kind equality for the type `t` it encodes.
  Which makes the *type application* (`TyApp`)
  give us an *arrow kind* for the Haskell *type constructor*
  it applies an Haskell type to, releaving me from tricky workarounds.
* `UndecidableInstances` to relax the checks that the type-level programming does terminate.

# Bugs

There are some of them hidding in there,
and the whole thing is far from being perfect…
Your comments, problem reports, or questions, are welcome!
You have my email address, so… just send me some emails :]

# To do

* Study to which point *type inferencing* is doable,
  now that `Type` is powerful enough to contain `TyVar`s.
* Study to which point error messages can be improved,
  now that there is a `Source` carried through all `Kind`s or `Type`s,
  it should enable some nice reports.
  Still, a lot of work and testing remain to be done,
  and likely some ideas to find too…
* Add more terms in [symantic-lib](https://hackage.haskell.org/package/symantic-lib).
* Add more transformations.
* Study how to list class instances.
* Study where to put `INLINE`, `INLINEABLE` or `SPECIALIZE` pragmas.
* Study how to support *rank-N polymorphic types*,
  special cases can likely use the *boxed polymorphism* workaround,
  but even if GHC were supporting *impredicative types*,
  I'm currently clueless about how to do this.