Write You a Haskell Stephen Diehl

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false = reserved ”false” >> return Fl zero = reservedOp ”0” >> return Zero expr :: Parser Expr expr = Ex.buildExpressionParser table factor factor :: Parser Expr factor = true false zero ifthen parens expr contents :: Parser a -> Parser a contents p = do Tok.whiteSpace lexer r Bool isVal Tr = True isVal Fl = True isVal t | isNum t = True isVal _ = False e evaluation of our languages uses the Maybe applicative to accommodate the fact that our reduction may halt at any level with a Nothing if the expression being reduced has reached a normal form or 40
cannot proceed because the reduction simply isn’t well-defined. e rules for evaluation are a single step by which an expression takes a single small step from one form to another by a given rule. eval’ x = case x of IsZero Zero -> Just Tr IsZero (Succ t) | isNum t -> Just Fl IsZero t -> IsZero (eval’ t) Succ t -> Succ (eval’ t) Pred Zero -> Just Zero Pred (Succ t) | isNum t -> Just t Pred t -> Pred (eval’ t) If Tr c _ -> Just c If Fl _ a -> Just a If t c a -> (\t’ -> If t’ c a) eval’ t _ -> Nothing At the toplevel we simply apply eval’ repeatedly until either a value is reached or we’re left with an expression that has no well-defined way to proceed. e term is “stuck” and the program is in an undefined state. nf x = fromMaybe x (nf eval’ x) eval :: Expr -> Maybe Expr eval t = case nf t of nft | isVal nft -> Just nft | otherwise -> Nothing -- term is ”stuck” REPL e driver for our simple language simply invokes all of the parser and evaluation logic in a loop feeding the resulting state to the next iteration. We will use the haskeline library to give us readline interactions for the small REPL. Behind the scenes haskeline is using readline or another platform-specific system library to manage the terminal input. To start out we just create the simplest loop, which only parses and evaluates expressions and prints them to the screen. We’ll build on this pattern in each chapter, eventually ending up with a more full-featured REPL. e two functions of note are the operations for the InputT monad transformer. runInputT :: Settings IO -> InputT IO a -> IO a getInputLine :: String -> InputT IO (Maybe String) When the user enters an EOF or sends a SIGQUIT to input, getInputLine will yield Nothing and can handle the exit logic. 41
Page 1 and 2: 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: module Syntax where data Expr = Tr
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 and 54: 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 and 106:
let rec fib n = if (n == 0) then 0
Page 107 and 108:
• Higher order functions • Para
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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