Write You a Haskell Stephen Diehl

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Design of ProtoHaskell Now that we’ve completed our simple little ML language, let’s discuss the road ahead toward building a more complex language we’ll call ProtoHaskell that will eventually become the full Fun language. Language Chapters Description Poly 1 - 8 Minimal type inferred ML-like language. ProtoHaskell 8 - 18 Interpreted minimal Haskell subset. Fun 18 - 27 ProtoHaskell with native code generator. e defining feature of ProtoHaskell is that is independent of an evaluation model, so hypothetically one could write either a lazy or strict backend and use the same frontend. Before we launch into writing compiler passes let’s look at the overview of where we’re going, the scope of what we’re going to do, and what needs to be done to get there. We will refer to concepts that are not yet introduced, so keep is meant to be referred to as a high-level overview of the ProtoHaskell compiler pipeline. Haskell: A Rich Language Haskell itself is a beautifully simple language at its core, although the implementation of GHC is arguably anything but simple! e more one digs into the implementation the more it becomes apparent that a lot of care and forethought was given to making the frontend language as expressive as it is. Many of these details require a great detail of engineering work to make them work as seamlessly as they do. Consider this simple Haskell example but note how much of an extension this is from our simple little ML interpreter. filter :: (a -> Bool) -> [a] -> [a] filter pred [] = [] filter pred (x:xs) | pred x = x : filter pred xs | otherwise = filter pred xs Consider all the things that are going on just in this simple example. • Lazy evaluation • Custom datatypes 104
• Higher order functions • Parametric polymorphism • Function definition by pattern matching • Pattern matching desugaring • Guards to distinguish sub-cases • Type signature must subsume inferred type • List syntactic sugar ( value/pattern syntax ) Clearly we’re going to need a much more sophisticated design, and we’ll likely be doing quite a bit more bookkeeping about our program during compilation. Scope Considering our project is intended to be a simple toy language, we are not going to implement all of Haskell 2010. Doing so in its entirety would actually be a fairly involved effort. However we will implement a sizable chunk of the functionality, certainly enough to write non-trivial programs and implement most of the standard Prelude. ings we will implement: • Indentation sensitive grammar • Pattern matching • Algebraic datatypes • Where statements • Recursive functions/datatypes • Operator sections • Implicit let-rec • List and tuple sugar • Records • Custom operators • Do-notation • Type annotations • Monadic IO • Typeclasses • Arithmetic primops • Type synonyms • List comprehensions ings we will not implement are: • Overloaded literals • GADTs • Polymorphic recursion 105
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Write You a Haskell Building a mode
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Contents Introduction 6 Goals . . .
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Evaluation . . . . . . . . . . . .
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Syntax Errors . . . . . . . . . . .
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• containers • unordered-contai
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File ””, line 1, in TypeError:
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] TokenSym ”x”, TokenAdd, Token
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f: mov res, arg add res, 1 ret defi
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Datatypes Constructors for datatype
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length :: [a] -> Int length [] = 0
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Higher-Kinded Types e “type of ty
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eturn a >>= f = f a Law 2 m >>= ret
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data PlatonicSolid = Tetrahedron |
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foo :: Stack () foo = Stack $ do pu
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, bar :: Int } deriving (Show) comp
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class IsString a where fromString :
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Parsing Parser Combinators For pars
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instance Monad Parser where return
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natural :: Parser Integer natural =
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print $ eval $ run a Now we can try
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module Syntax where data Expr = Tr
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cannot proceed because the reductio
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later into native CPU instructions
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λx.e ere are several syntactical c
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type Name = String data Expr = Var
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Eta-expansion (λx.ex)e ′ β →
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Y = SSK(S(K(SS(S(SSK))))K) In a unt
Page 55 and 56: parensIf :: Bool -> Doc -> Doc pare
Page 57 and 58: In this notation, the expression ab
Page 59 and 60: In all the languages which we will
Page 61 and 62: eval1 expr = case expr of Succ t ->
Page 63 and 64: TNat -> return TNat _ -> throwError
Page 65 and 66: • T-Var Variables are simply pull
Page 67 and 68: t1 throwError $ Mismatch t2 a ty -
Page 69 and 70: Full Source • Typed Arithmetic
Page 71 and 72: 1. Evaluate f to λy.e 2. Evaluate
Page 73 and 74: e 1 → e ′ 1 e 1 e 2 → e ′ 1
Page 75 and 76: case we will use a GADT to embed a
Page 77 and 78: AppP (AppP (VarP f) (VarP x)) (AppP
Page 79 and 80: e prim function will simply perform
Page 81 and 82: ere is nothing more practical than
Page 83 and 84: As before let rec expressions will
Page 85 and 86: Γ, x : τ = (Γ\x) ∪ {x : τ} Op
Page 87 and 88: where s’ = foldr Map.delete s as
Page 89 and 90: s2 Type -> Infer Subst bind a t |
Page 91 and 92: infer :: TypeEnv -> Expr -> Infer (
Page 93 and 94: App e1 e2 -> do tv
Page 95 and 96: -- | Unify two types uni :: Type ->
Page 97 and 98: unifies t (TVar v) = v ‘bind‘ t
Page 99 and 100: Interpreter Our evaluator will oper
Page 101 and 102: Our language can be compiled into a
Page 103 and 104: exec True contents -- :type command
Page 105: let rec fib n = if (n == 0) then 0
Page 109 and 110: • e PHOAS, the type-erased Core i
Page 111 and 112: If the ProtoHaskell compiler is inv
Page 113 and 114: infixl 6 + infixl 7 * f = 1 + 2 * 3
Page 115 and 116: -- Desugared f x = case x of Just _
Page 117 and 118: data Kind = KStar | KArr Kind Kind
Page 119 and 120: TypeError) -- | Inference state dat
Page 121 and 122: e expression or Expr type is the co
Page 123 and 124: data qname [var] where [tydecl] dat
Page 125 and 126: Typeclass Declarations Typeclass de
Page 127 and 128: Syntax Name Kind Description (,) Pa
Page 129 and 130: So for instance if we have three AS
Page 131 and 132: -- **Begin Haskell Syntax** { {-# O
Page 133 and 134: tokens :- -- Whitespace insensitive
Page 135 and 136: Expr : let VAR ’=’ Expr in Expr
Page 137 and 138: data Type = TVar Loc TVar | TCon Lo
Page 139 and 140: -- Start of layout ( Column: 0 ) fi
Page 141 and 142: Extensible Operators Haskell famous
Page 143 and 144: fixityspec :: Parser FixitySpec fix
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Write You a Haskell Stephen Diehl

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