goals:
  simple, clean API and dependency tree

stage 1:
typesig - LambdaCube.Compiler.Utils
import LambdaCube.Compiler.Pretty
import LambdaCube.Compiler.DeBruijn

typesig - import LambdaCube.Compiler.Lexer
typesig - import LambdaCube.Compiler.DesugaredSource
typesig - import LambdaCube.Compiler.Patterns
typesig - import LambdaCube.Compiler.Statements
typesig - import LambdaCube.Compiler.Parser

stage 2:
import LambdaCube.Compiler.Core
import LambdaCube.Compiler.Infer
import LambdaCube.Compiler.InferMonad

stage 3:
import LambdaCube.Compiler.CoreToIR

IDEA:
  add each module a Pattern module with the pattern synonyms that provides the rich API

problem:
  low level use of debruijn indices
  too many and indiscpilined pattern synonyms


type classes:
  Debruijn:         Rearrange, HasFreeVars, DeBruijnify
  Pretty:           PShow
  Patterns:         PatVars
  DesugaredSource:  SourceInfo, SetSourceInfo

  Core:             HasFName, Subst, MkDoc, NType

data types:
  Lexer:            ParseEnv, Namespace
  Parser:           LCParseError, ParseWarning, DesugarInfo
  Pretty:           Fixity, Doc, DocAtom
  Patterns:         ParseCheck, Pat_ c, ParPat_ c, Lets a, GuardTree
  DesugaredSource:  FileInfo, Range, SI, SIName, FNameTag, Lit, SExp' a, Binder, Visibility, Stmt, Module_ a, Export, ImportItems, Extension
  Utils:            Void
  Statements:       PreStmt, StmtNode

  Infer:            CEnv, Env
  InferMonad:       ErrorMsg, Info
  Core:             ConName, TyConName, FunName, CaseFunName, TyCaseFunName, FunDef, Freq, Exp, Neutral, ExpType

newtypes:
  Lexer:            ParseError
  DeBruijn:         FreeVars
  DesugaredSource:  SPos, FName
  Utils:            SData

patterns:
  Pretty:           DText, DAt, DApp, DHApp, DSemi, DArr_, DCstr, DAnn, DLet, DComma, DPar, DParen, DBrace, DBracket, DOp, DOp0, DSep, DGlue, DArr, DForall,
                    DContext', DContext, DParContext, DLam, DLet'
  Patterns:         ParPat, PWildcard, PCon, ViewPat, PatType, PVarSimp, PConSimp, ViewPatSimp, PatTypeSimp, PParens, Otherwise
  DesugaredSource:  Ticked, CaseName, MatchName, SPos, SIName_, SIName, Tag, FTag, SBind, SPi, SLam, Wildcard, SLet, SLamV, SVar, SApp, SAppH, SAppV, SAppV2,
                    SBuiltin, SRHS, Section, SType, SConstraint, Parens, SAnn, SCW, TyType, HList, HCons, HNil, BList, BCons, BNil, UncurryS, AppsS, StLet, Primitive

  Infer:            EBind2, CheckType
  Core:             TType, NoRHS, Fun, CaseFun_, TyCaseFun_, App_, Con, TyCon, Lam, Pi, Let, SubstLet, CaseFun, TyCaseFun, Var, App, DFun, DFun_, UFun, DFunN,
                    DFunN_, ConN, TyConN, TTyCon0, :~>, TConstraint, Unit, TInt, TNat, TBool, TFloat, TString, TChar, TOrdering, TVec, Empty, TT, CUnit, CEmpty,
                    CstrT, CstrT', Coe, ParEval, T2, CW, CW_, CSplit,HLit, HInt, HFloat, HChar, HString, EBool, ENat, ReducedN, Reduced, FFix, NonNeut

Other material:
closure conversion tutorial: https://gist.github.com/jozefg/652f1d7407b7f0266ae9
inference: https://github.com/jozefg/higher-order-unification/blob/master/explanation.md
           https://www.reddit.com/r/haskell/comments/6rs1bp/a_tutorial_on_implementing_higherorder/