haskell

问题 I'm trying to use a free monad to build an EDSL for constructing AND/OR decision trees like Prolog, with >>= mapped to AND, and mplus mapped to OR. I want to be able to describe something like A AND (B OR C) AND (D OR E) , but I don't want distributivity to turn this into (A AND B AND D) OR (A AND B AND E) OR (A AND C AND D) OR (A AND C AND E) . Ultimately, I want to transform the AND/OR nodes into reified constraints in a constraint solver, without causing the combinatorial explosion in the

问题 I have been working on a pet language, which has Haskell-like syntax. One of the neat things which Haskell does, which I have been trying to replicate, is its insertion of {, } and ; tokens based on code layout, before the parsing step. I found http://www.haskell.org/onlinereport/syntax-iso.html, which includes a specification of how to implement the layout program, and have made a version of it (modified, of course, for my (much simpler) language). Unfortunately, I am getting an incorrect

问题 I have been working on a pet language, which has Haskell-like syntax. One of the neat things which Haskell does, which I have been trying to replicate, is its insertion of {, } and ; tokens based on code layout, before the parsing step. I found http://www.haskell.org/onlinereport/syntax-iso.html, which includes a specification of how to implement the layout program, and have made a version of it (modified, of course, for my (much simpler) language). Unfortunately, I am getting an incorrect

问题 I want to make a superclass of Num, called Linear class Linear a where add :: a -> a -> a instance (Num a) => Linear a where add = (+) I get the error : Illegal instance declaration for `Linear a' (All instance types must be of the form (T a1 ... an) where a1 ... an are *distinct type variables*, and each type variable appears at most once in the instance head. Use -XFlexibleInstances if you want to disable this.) In the instance declaration for `Linear a' From what I understand, something

问题 First question ever here, and newbie in both Common LISP and Haskell, please be kind. I have a function in Common LISP - code below - which is intended to tell whether the area of a triangle is an integral number (integer?). (defun area-int-p (a b c) (let* ((s (/ (+ a b c) 2)) (area (sqrt (* s (- s a) (- s b) (- s c))))) (if (equal (ceiling area) (floor area)) t nil))) This is supposed to use Heron's formula to calculate the area of the triangle, given the size of the three sides, and decide

问题 For example, given a value, v , and a function f , is there a way to get (f v,v) point free? 回答1: Alternatively, note that for functions h and f with appropriate types, h >>= f = \w -> f (h w) w so you can write f >>= (,) 回答2: import Control.Arrow (g &&& f) v = (g v, f v) -- ergo, (id &&& f) v = (v, f v) (f &&& id) v = (f v, v) 回答3: How about using the Applicative instance for (->) liftA2 (,) id :: (a -> b) -> a -> (a, b) For example liftA2 (,) id succ 5 >>> (5,6) 来源： https://stackoverflow

问题 This question already has answers here : Decrementing ranges in Haskell (3 answers) Closed 4 years ago . Why in Haskell is not working range downto without step [7..1] => [] but working only this [7,6..1] => [7,6,5,4,3,2,1] 回答1: 3.10. Arithmetic sequences [...] Arithmetic sequences satisfy these identities: [...] [ e1..e3 ] = enumFromTo e1 e3 [...] 6.3.4 The Enum Class For the types Int and Integer, the enumeration functions have the following meaning: [...] The sequence enumFromTo e1 e3 is

问题 This question already has answers here : Decrementing ranges in Haskell (3 answers) Closed 4 years ago . Why in Haskell is not working range downto without step [7..1] => [] but working only this [7,6..1] => [7,6,5,4,3,2,1] 回答1: 3.10. Arithmetic sequences [...] Arithmetic sequences satisfy these identities: [...] [ e1..e3 ] = enumFromTo e1 e3 [...] 6.3.4 The Enum Class For the types Int and Integer, the enumeration functions have the following meaning: [...] The sequence enumFromTo e1 e3 is

问题 Haskell can derive the instance for MonadState s in T1 below but not in T2 which is however a very similar type. In which way should I modify the code for T2 so that the instance for MonadState s can be automatically derived? {-# LANGUAGE GeneralizedNewtypeDeriving #-} import Control.Monad.Reader import Control.Monad.State newtype T1 r s a = T1 { runT1 :: ReaderT r (State s) a } deriving (Monad, MonadReader r, MonadState s) newtype T2 r s a = T2 { runT2 :: StateT r (State s) a } deriving

问题 Haskell can derive the instance for MonadState s in T1 below but not in T2 which is however a very similar type. In which way should I modify the code for T2 so that the instance for MonadState s can be automatically derived? {-# LANGUAGE GeneralizedNewtypeDeriving #-} import Control.Monad.Reader import Control.Monad.State newtype T1 r s a = T1 { runT1 :: ReaderT r (State s) a } deriving (Monad, MonadReader r, MonadState s) newtype T2 r s a = T2 { runT2 :: StateT r (State s) a } deriving