Write You a Haskell Stephen Diehl

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eturn (TArr l (setLoc l ty) tv) Lit l _ -> return (setLoc l typeInt) Now specifically at the call site of our unification solver, if we encounter a unification fail we simply pluck the location information off the two type terms and plug it into the type error fields. unifies t1 t2 | t1 == t2 = return emptyUnifer unifies (TVar _ v) t = v ‘bind‘ t unifies t (TVar _ v) = v ‘bind‘ t unifies (TArr _ t1 t2) (TArr _ t3 t4) = unifyMany [t1, t2] [t3, t4] unifies (TCon _ a) (TCon _ b) | a == b = return emptyUnifer unifies t1 t2 = throwError $ UnificationFail t1 (getLoc t1) t2 (getLoc t2) bind :: TVar -> Type -> Solve Unifier bind a t | eqLoc t a = return (emptySubst, []) | occursCheck a t = throwError $ InfiniteType a (getLoc t) t | otherwise = return $ (Subst $ Map.singleton a t, []) So now we can explicitly trace the provenance of the specific constraints that gave rise to a given type error all the way back to the source that generated them. Cannot unify types: Int Introduced at line 27 column 5 f 2 3 with Int -> c Introduced at line 5 column 9 let f x y = x y is is of course the simplest implementation of the this tracking method and could be further extended by giving an weighted ordering to the constraints based on their likelihood of importance and proximity and then choosing which location to report based on this information. is remains an open area of work. Indentation <strong>Haskell</strong>’s syntax uses indentation blocks to delineated sections of code. is use of indentation sensitive layout to convey the structure of logic is sometimes called the offside rule in parsing literature. At the beginning of “laidout” block the first declaration or definition can start in any column, and the parser marks that indentation level. Every subsequent top-level declaration must have the same indentation. 136
-- Start of layout ( Column: 0 ) fib :: Int -> Int fib x = truncate $ ( 1 / sqrt 5 ) * ( phi ^ x - psi ^ x ) -- (Column: > 0) -- Start of new layout ( Column: 2 ) where -- Indented block ( Column: > 2 ) phi = ( 1 + sqrt 5 ) / 2 psi = ( 1 - sqrt 5 ) / 2 e Parsec monad is itself parameterized over a type variable s which stands for the State layer baked into the monad allowing us to embed custom parser state inside of our rules. To adopt our parser to handle sensitive whitespace we will -- Indentation sensitive Parsec monad. type IParsec a = Parsec Text ParseState a data ParseState = ParseState { indents :: Column } deriving (Show) initParseState :: ParseState initParseState = ParseState 0 Inside of the Parsec the internal position state (SourcePos) is stored during each traversal, and is accessible inside of rule logic via getPosition function. data SourcePos = SourcePos SourceName !Line !Column getPosition :: Monad m => ParsecT s u m SourcePos In terms of this function we can write down a set of logic that will allow us to query the current column count and then either succeed or fail to match on a pattern based on the current indentation level. e laidout combinator will capture the current indentation state and push it into the indents field in the State monad. laidout :: Parsec s ParseState a -> Parsec s ParseState a laidout m = do cur
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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
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parensIf :: Bool -> Doc -> Doc pare
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In this notation, the expression ab
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In all the languages which we will
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eval1 expr = case expr of Succ t ->
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TNat -> return TNat _ -> throwError
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• T-Var Variables are simply pull
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t1 throwError $ Mismatch t2 a ty -
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Full Source • Typed Arithmetic
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1. Evaluate f to λy.e 2. Evaluate
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e 1 → e ′ 1 e 1 e 2 → e ′ 1
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case we will use a GADT to embed a
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AppP (AppP (VarP f) (VarP x)) (AppP
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e prim function will simply perform
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ere is nothing more practical than
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As before let rec expressions will
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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: data Type = TVar Loc TVar | TCon Lo
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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