symantic/README.md

   1 # symantic
   2
   3 This is an experimental library for composing, parsing, typing, compiling, transforming and interpreting
   4 a custom DSL (Domain-Specific Language).
   5
   6 # Features
   7
   8 Those custom DSL can express a subset of GHC's Haskell type system:
   9
  10 * *first class functions* (aka. *lambdas*),
  11 * chosen *monomorphic types* (like `Bool` or `Maybe`),
  12 * chosen *rank-1 polymorphic types* (like `(Maybe a)`),
  13 * chosen *type class instances*,
  14 * chosen *type family instances*,
  15 * and chosen *type constraints*;
  16
  17 where "chosen X" means declared in Haskell
  18 and selected when composing the DSL.
  19
  20 In particular, this library is currently not able to:
  21
  22 * do *type inferencing* for the argument of *lambdas*
  23   (they must all be explicitely annotated, aka. *Church-style*),
  24 * do *pattern matching* (aka. *case*) (but *Church-encoding* functions are often enough),
  25 * do *rank-N polymorphic types* (aka. *non-prenex forall*, like `(forall s. ST s a) -> a`).
  26
  27 And by itself, the DSL is only able to define new terms to be interpreted,
  28 no new types, or other type-level structures.
  29
  30 # Warning
  31
  32 Please be aware that despite its using of powerful ideas from clever people,
  33 this remains a FUND-LESS SINGLE-PERSON EXPERIMENTAL LIBRARY.
  34 Meaning that it IS NOT heavily tested and documented,
  35 DOES NOT have a strong commitment to preserving backward compatibility,
  36 MAY FAIL to comply with the [PVP](http://www.haskell.org/haskellwiki/Package_versioning_policy),
  37 and CAN die without notice.
  38 You've been warned.
  39
  40 # Use cases
  41
  42 The main goal of this library is to enable the runtime interpretation of terms,
  43 type-checked according to some types defined at composing-time (ie. GHC's compile-time).
  44
  45 Using a DSL enables to limit expressiveness in order to ease analysis.
  46 Here the idea is that the more complex logic shall remain written in Haskell,
  47 and then this library used to project an interface into a DSL
  48 (using GHC's Haskell as a FFI (Foreign Function Interface)).
  49 This in order to give runtime users the flexibility
  50 to write programs not requiring a full-blown Haskell compiler,
  51 yet enabling enough flexibility to let them express complex needs
  52 with a reasonably advanced type-safe way
  53 and a controlled environment of primitives.
  54
  55 ## Typical use cases
  56
  57 * Enabling runtime users to enter some Haskell-like expressions
  58   (maybe with a more convenient syntax wrt. the domain problem)
  59   without using at runtime all the heavy machinery and ecosystem of GHC
  60   (eg. by using [hint](https://hackage.haskell.org/package/hint)),
  61   but still leaning on primitive functions coded in GHC's Haskell.
  62 * Limiting those expressions to be built from well-controlled expressions only.
  63 * Run some analyzes/optimizations on those well-controlled expressions.
  64 * Report errors specific to the domain problem.
  65
  66 # Usage
  67
  68 Please learn how to use this library by reading example source files in `test/`
  69 in [symantic-lib](https://hackage.haskell.org/package/symantic-lib)
  70 in [Git repository](git://git.autogeree.net/symantic).
  71
  72 These `test` files use [megaparsec](https://hackage.haskell.org/package/megaparsec) as parser
  73 (see `test/Testing/Megaparsec.hs`) and a default grammar somehow sticking to Haskell's,
  74 but staying context-free (so in particular: insensitive to the indentation),
  75 and supporting prefix and postfix operators.
  76 This grammar — itself written as a symantic embedded DSL
  77 with [symantic-grammar](https://hackage.haskell.org/package/symantic-grammar) —
  78 can be reused to build other ones, is not bound to a specific parser,
  79 and can produce its own EBNF rendition.
  80
  81 # Acknowledgements
  82
  83 This library would probably be much worse than it is
  84 without the following seminal works:
  85
  86 * [Finally Tagless](http://okmij.org/ftp/tagless-final/) by Jacques Carette, Oleg Kiselyov, and Chung-chieh Shan.
  87 * [Dependent Types in Haskell](http://cs.brynmawr.edu/~rae/papers/2016/thesis/eisenberg-thesis.pdf) by Richard A. Eisenberg.
  88 * [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.
  89 * [Typeable](https://ghc.haskell.org/trac/ghc/wiki/Typeable) by Ben Gamari and others.
  90
  91 # Main ideas
  92
  93 * __Symantic DSL__.
  94   Terms are encoded in the [Tagless-Final](http://okmij.org/ftp/tagless-final/) way (aka. the *symantic* way)
  95   which leverages the *type class* system of Haskell — instead of using *data types* — to form an embedded DSL.
  96   More specifically, a *class* encodes the *syntax* of terms (eg. `Sym_Bool`)
  97   and its *class instances* on a dedicated type encodes their *semantics*
  98   (eg. `(Sym_Bool Eval)` interprets a term as a value of its type
  99   in the host language (`Bool` in Haskell here),
 100   or `(Sym_Bool View)` interprets a term as a textual rendition, etc.).
 101
 102   DSL are then composed/extended by selecting those symantic *classes*
 103   (and in an embedded DSL those could even be automatically inferred,
 104   when `NoMonomorphismRestriction` is on).
 105   Otherwise, when using symantics for a non-embedded DSL
 106   — the whole point of this library — the *classes* composing the DSL
 107   are selected manually at GHC's compile-time,
 108   through the *type-level list* `ss` given to `readTerm`.
 109
 110   Moreover, those symantic `term`s are parameterized by the type of the value they encode,
 111   in order to get the same type safety as with plain Haskell values.
 112   Hence the symantic *classes* have the higher kind: `((* -> *) -> Constraint)`
 113   instead of just `(* -> Constraint)`.
 114
 115   Amongst those symantics, `Sym_Lambda` introduces *lambda abstractions* by an higher-order approach,
 116   meaning that they directly reuse GHC's internal machinery
 117   to abstract or instantiate variables,
 118   which I think is by far the most efficient and simplest way of doing it
 119   (no (generalized or not) DeBruijn encoding
 120   like in [bound](https://hackage.haskell.org/package/bound)'s `Monad`s).
 121
 122 * __Singleton for any type__.
 123   To typecheck terms using a `(Type src vs t)` which acts as a *singleton type*
 124   for any Haskell *type index* `t` of any kind,
 125   which is made possible with the dependant Haskell extensions:
 126   especially `TypeFamilies`, `GADTs` and `TypeInType`.
 127
 128 * __Type constants using `Typeable`__.
 129   *Type constant* could be introduced by indexing them amongst a *type-level list*,
 130   but since they are *monomorphic types*, using `Typeable` simplifies
 131   the machinery, and is likely more space/time efficient, including at GHC-compile-time.
 132
 133 * __Type variables using a type-level list__.
 134   Handling *type variables* is done by indexing them amongst a `vs` *type-level list*,
 135   where each *type variable* is wrapped inside a `Proxy` to handle different kinds.
 136   Performing a substitution (in `substVar`) preserves the *type index* `t`,
 137   which is key for preserving any associated `Term`.
 138   Unifying *type variables* is done with `unsafeCoerce` (in `unifyType`),
 139   which I think is necessary and likely safe.
 140
 141 # Main extensions
 142
 143 * `AllowAmbiguousTypes` for avoiding a lot of uses of `Proxy`.
 144 * `ConstraintKinds` for *type lists* to contain `Constraint`s,
 145   or reifying any `Constraint` as an explicit dictionary `Dict`,
 146   or defining *type synonym* of *type classes*,
 147   or merging *type constraints*.
 148 * `DataKinds` for type-level data structures (eg. *type-level lists*).
 149 * `DefaultSignatures` for providing identity transformations of terms,
 150   and thus avoid boilerplate code when a transformation
 151   does not need to alter all semantics.
 152   Almost as explained in [Reducing boilerplate in finally tagless style](https://ro-che.info/articles/2016-02-03-finally-tagless-boilerplate).
 153 * `GADTs` for knowing types by pattern-matching terms,
 154   or building terms by using type classes.
 155 * `PolyKinds` for avoiding a lot of uses of `Proxy`.
 156 * `Rank2Types` or `ExistentialQuantification` for parsing `GADT`s.
 157 * `TypeApplications` for having a more concise syntax
 158   to build `Type` (eg. `tyConst @Bool`).
 159 * `TypeFamilies` for type-level programming.
 160 * `TypeInType` (introduced in GHC 8.0.1)
 161   for `Type` to also bind a kind equality for the type `t` it encodes.
 162   Which makes the *type application* (`TyApp`)
 163   give us an *arrow kind* for the Haskell *type constructor*
 164   it applies an Haskell type to, releaving me from tricky workarounds.
 165 * `UndecidableInstances` to relax the checks that the type-level programming does terminate.
 166
 167 # Bugs
 168
 169 There are some of them hidding in there,
 170 and the whole thing is far from being perfect…
 171 Your comments, problem reports, or questions, are welcome!
 172 You have my email address, so… just send me some emails :]
 173
 174 # To do
 175
 176 * Study to which point *type inferencing* is doable,
 177   now that `Type` is powerful enough to contain `TyVar`s.
 178 * Study to which point error messages can be improved,
 179   now that there is a `Source` carried through all `Kind`s or `Type`s,
 180   it should enable some nice reports.
 181   Still, a lot of work and testing remain to be done,
 182   and likely some ideas to find too…
 183 * Add more terms in [symantic-lib](https://hackage.haskell.org/package/symantic-lib).
 184 * Add more transformations.
 185 * Study how to list class instances.
 186 * Study where to put `INLINE`, `INLINEABLE` or `SPECIALIZE` pragmas.
 187 * Study how to support *rank-N polymorphic types*,
 188   special cases can likely use the *boxed polymorphism* workaround,
 189   but even if GHC were supporting *impredicative types*,
 190   I'm currently clueless about how to do this.