You could always just use Data.Sequence. Alternatively, a well-known implementation of a purely functional queue is to use two lists. One for enqueue and another for dequeue. Enqueue would simply cons with the enqueue list. Dequeue takes the head of the dequeue list. When the dequeue list is shorter than the enqueue list, refill it … Read more
The usual way to convert an Int to a Double is to use fromIntegral, which has the type (Integral a, Num b) => a -> b. This means that it converts an Integral type (Int and Integer) to any numeric type b, of which Double is an instance. Your case sounds like you want to … Read more
!! indexes lists. It takes a list and an index, and returns the item at that index. If the index is out of bounds, it returns ⊥.
It means not equal. So 5 /= 5 is false as 5 == 5 is true. x /= y = not (x == y) As suggested, it recalls the mathematical symbol “≠” (/=) opposite to “=” (==).
The answer depends mostly on whether the foreign call is a safe or an unsafe call. An unsafe C call is basically just a function call, so if there’s no (nontrivial) type conversion, there are three function calls if you make three foreign calls, and between one and four when you write a wrapper in … Read more
The typechecker doesn’t know when to apply the subsumption rule. You can tell it when with the following function. Prelude> let u :: ((f a -> f a) -> c) -> ((forall b. f b -> f b) -> c); u f n = f n This says, given a function from a transformation for … Read more
The Haskell Platform specifies mtl and transformers as standard. If you’re unsure, you should just use mtl. However, if you have a specific technical reason to look at the new libraries, they tend to address issues or add new features to mtl. monadLib in particular has some new features.
Please check out the numbers package. If all you need is to store exact numbers like “1 + √3”, you may want to use Data.Number.CReal instead of symbolic arithmetics. It stores the expressions and can be computed to arbitrary number of digits when needed. Prelude Data.Number.CReal> let cx = 1 + sqrt (3 :: CReal) … Read more
That at and ix are different is already noticable if you look at the available instances for the classes containing these functions: instances of At: Map, IntMap, HashMap instances of Ixed: [a], Map, ByteString, Text, and a lot more All instances if At are also an instance of Ix, but not all instances of Ix … Read more
Any file descriptor that can be managed by epoll/kqueue is eligible. Libraries that want asynchronous treatment of I/O need to cooperate with the I/O manager by making file descriptors non-blocking, and calling the threadWaitRead and threadWaitWrite functions in GHC.Conc before retrying a system call that previously returned EWOULDBLOCK. This has already been done for the … Read more