Write You a Haskell Stephen Diehl

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Pretty Printing Hackage provides quite a few pretty printer libraries that ease the process of dumping out textual forms for our data types. Although there are some differences between the libraries most of them use the same set of combinators. We will use the Text.PrettyPrint module from the pretty package on Hackage. Most of our pretty printing will be unavoidable boilerplate but will make debugging internal state much easier. Combinators Concatenation Spaced concatenation char Renders a character as a Doc text Renders a string as a Doc hsep Horizontally concatenates a list of Doc vcat Vertically joins a list of Doc with newlines e core type of the pretty printer is the Doc type which is the abstract type of documents. Combinators over this type will manipulate the internal structure of this document which is then finally reified to an physical string using the render function. Since we intend to pretty print across multiple types we will create a Pretty typeclass. module Pretty where import Text.PrettyPrint class Pretty p where ppr :: Int -> p -> Doc pp :: p -> Doc pp = ppr 0 First, we create two helper functions that collapse our lambda bindings so we can print them out as single lambda expressions. viewVars :: Expr -> [Name] viewVars (Lam n a) = n : viewVars a viewVars _ = [] viewBody :: Expr -> Expr viewBody (Lam _ a) = viewBody a viewBody x = x en we create a helper function for parenthesizing subexpressions. 52
parensIf :: Bool -> Doc -> Doc parensIf True = parens parensIf False = id Finally, we define ppr. e p variable will indicate our depth within the current structure we’re printing and allow us to print out differently to disambiguate it from its surroundings if necessary. instance Pretty Expr where ppr p e = case e of Lit (LInt a) -> text (show a) Lit (LBool b) -> text (show b) Var x -> text x App a b -> parensIf (p>0) $ (ppr (p+1) a) (ppr p b) Lam x a -> parensIf (p>0) $ char ’\\’ hsep (fmap pp (viewVars e)) ”->” ppr (p+1) (viewBody e) ppexpr :: Expr -> String ppexpr = render . ppr 0 Full Source • Untyped Lambda Calculus 53
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Write You a Haskell Building a mode
Page 3 and 4: Contents Introduction 6 Goals . . .
Page 5 and 6: Evaluation . . . . . . . . . . . .
Page 7 and 8: Syntax Errors . . . . . . . . . . .
Page 9 and 10: • containers • unordered-contai
Page 11 and 12: File ””, line 1, in TypeError:
Page 13 and 14: ] TokenSym ”x”, TokenAdd, Token
Page 15 and 16: f: mov res, arg add res, 1 ret defi
Page 17 and 18: Datatypes Constructors for datatype
Page 19 and 20: length :: [a] -> Int length [] = 0
Page 21 and 22: Higher-Kinded Types e “type of ty
Page 23 and 24: eturn a >>= f = f a Law 2 m >>= ret
Page 25 and 26: data PlatonicSolid = Tetrahedron |
Page 27 and 28: foo :: Stack () foo = Stack $ do pu
Page 29 and 30: , bar :: Int } deriving (Show) comp
Page 31 and 32: class IsString a where fromString :
Page 33 and 34: Parsing Parser Combinators For pars
Page 35 and 36: instance Monad Parser where return
Page 37 and 38: natural :: Parser Integer natural =
Page 39 and 40: print $ eval $ run a Now we can try
Page 41 and 42: module Syntax where data Expr = Tr
Page 43 and 44: cannot proceed because the reductio
Page 45 and 46: later into native CPU instructions
Page 47 and 48: λx.e ere are several syntactical c
Page 49 and 50: type Name = String data Expr = Var
Page 51 and 52: Eta-expansion (λx.ex)e ′ β →
Page 53: Y = SSK(S(K(SS(S(SSK))))K) In a unt
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 and 106:
let rec fib n = if (n == 0) then 0
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• Higher order functions • Para
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• e PHOAS, the type-erased Core i
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If the ProtoHaskell compiler is inv
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infixl 6 + infixl 7 * f = 1 + 2 * 3
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-- Desugared f x = case x of Just _
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data Kind = KStar | KArr Kind Kind
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TypeError) -- | Inference state dat
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e expression or Expr type is the co
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data qname [var] where [tydecl] dat
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Typeclass Declarations Typeclass de
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Syntax Name Kind Description (,) Pa
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So for instance if we have three AS
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-- **Begin Haskell Syntax** { {-# O
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tokens :- -- Whitespace insensitive
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Expr : let VAR ’=’ Expr in Expr
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data Type = TVar Loc TVar | TCon Lo
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-- Start of layout ( Column: 0 ) fi
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Extensible Operators Haskell famous
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fixityspec :: Parser FixitySpec fix
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Write You a Haskell Stephen Diehl

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