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J.A. Alonso

Programming errors in traversal programs over structured data - 0 views

userpages.uni-koblenz.de/...syb42

artículo Haskell

shared by J.A. Alonso on 30 Jan 12 - No Cached
  • J.A. Alonso on 30 Jan 12
     
    Traversal strategies `a la Stratego (also `a la Strafunski and 'Scrap Your Boilerplate') provide an exceptionally versatile and uniform means of querying and transforming deeply nested and heterogeneously structured data including terms in functional programming and rewriting, objects in OO programming, and XML documents in XML programming. However, the resulting traversal programs are prone to programming errors. We are specifically concerned with errors that go beyond conservative type errors; examples we examine include divergent traversals, prematurely terminated traversals, and traversals with dead code. Based on an inventory of possible programming errors we explore options of static typing and static analysis so that some categories of errors can be avoided. This exploration generates suggestions for improvements to strategy libraries as well as their underlyingq programming languages. Haskell is used for illustrations and specifications with sufficient explanations to make the presentation comprehensible to the non-specialist. The overall ideas are language-agnostic and they are summarized accordingly.
Javier Neira

Understanding Monads Via Python List Comprehensions « All Unkept - 0 views

lukeplant.me.uk/...via-python-list-comprehensions

haskell monads tutorial python

shared by Javier Neira on 02 Mar 10 - Cached
  • But here we have taken it to a higher level -- the Monad interface is like an abstraction of any kind of container.
  • This in turn leads to the concept that a monadic value represents a computation -- a method for computing a value, bound together with its input value.
  • Writing monads is hard, but it pays off as using them in Haskell is surprisingly easy, and allows you to do some very powerful things.
  • ...3 more annotations...
  • One of them you have seen explicitly -- it's the 'return' method, responsible for packing things up into the monad. The other is called 'bind' or '>>=', and it does the 'unpacking' involved with the <- arrow in the do notation.
  • the 'bind' method doesn't really unpack and return the data. Instead, it is defined in such a way that it handles all unpacking 'internally', and you have to provide functions that always have to return data inside the monad.
  • It looks very much like 'unpack this data from the monad so I can use it', so it helps conceptually. In fact, together with the rest of the body of the 'do' block it forms an anonymous lambda function,
Javier Neira

Applicative-what? Functor-who? « wxfz :: Blog - 0 views

wxfz.wordpress.com/...applicative-what-functor-who

haskell control structures functors arrows monads intro

shared by Javier Neira on 09 Sep 10 - No Cached
  • Monads Are  a kind of abstract data type used to represent computations (instead of data in the domain model).
mountain

DData - 3 views

www.cs.uu.nl/ddata.html

del.icio.us haskell imported

shared by mountain on 24 Jun 07 - Cached
N N liked it
  • mountain on 24 Jun 07
     
    basic data structure in haskell
Javier Neira

A Neighborhood of Infinity: Haskell Monoids and their Uses - 0 views

blog.sigfpe.com/...ll-monoids-and-their-uses.html

monoids haskell monads examples howto

shared by Javier Neira on 30 Dec 09 - Cached
  • The Writer MonadYou can think of monoids as being accumulators. Given a running total, n, we can add in a new value a to get a new running total n' = n `mappend` a. Accumulating totals is a very common design pattern in real code so it's useful to abstract this idea. This is exactly what the Writer monad allows. We can write monadic code that accumulates values as a "side effect". The function to perform the accumulation is (somewhat confusingly) called tell. Here's an example where we're logging a trace of what we're doing.
  • This is an implementation of the factorial function that tells us what it did.
  • We use runWriter to extract the results back out. If we run> ex1 = runWriter (fact1 10)we get back both 10! and a list of what it took to compute this.
  • ...6 more annotations...
  • and the monoid for addition is Sum
  • but there is a big advantage to using the Writer version. It has type signature f :: Integer -> Writer (Sum Integer) Integer. We can immediately read from this that our function has a side effect that involves accumulating a number in a purely additive way.
  • This is the Bool type with the disjunction operator, better known as ||.
  • "tell my caller if any value of r is ever 120"
  • One last application to mention is the Data.Foldable library. This provides a generic approach to walking through a datastructure, accumulating values as we go. The foldMap function applies a function to each element of our structure and then accumulates the return values of each of these applications. An implementation of foldMap for a tree structure might be
  • Suppose we want to accumulate two side effects at the same time. For example, maybe we want to both count instructions and leave a readable trace of our computation. We could use monad transformers to combine two writer monads. But there is a slightly easier way - we can combine two monoids into one 'product' monoid. It's defined like this:instance (Monoid a,Monoid b) => Monoid (a,b) where mempty = (mempty,mempty) mappend (u,v) (w,x) = (u `mappend` w,v `mappend` x)
Javier Neira

Existential type - HaskellWiki - 0 views

www.haskell.org/...Existential_type

ghc type existential

shared by Javier Neira on 21 Dec 09 - Cached
  • First of all, it is now impossible for a function to demand a Worker having a specific type of buffer. Second, the type of foo can now be derived automatically without needing an explicit type signature. (No monomorphism restriction.) Thirdly, since code now has no idea what type the buffer function returns, you are more limited in what you can do to it.
  • This illustrates creating a heterogeneous list, all of whose members implement "Show", and progressing through that list to show these items: data Obj = forall a. (Show a) => Obj a   xs :: [Obj] xs = [Obj 1, Obj "foo", Obj 'c']   doShow :: [Obj] -> String doShow [] = "" doShow ((Obj x):xs) = show x ++ doShow xs With output: doShow xs ==> "1\"foo\"'c'"
  • Existential types in conjunction with type classes can be used to emulate the dynamic dispatch mechanism of object oriented programming languages. To illustrate this concept I show how a classic example from object oriented programming can be encoded in Haskell.
Javier Neira

99 questions/1 to 10 - HaskellWiki - 0 views

www.haskell.org/...1_to_10

haskell questions howto 99

shared by Javier Neira on 10 Nov 09 - Cached
  • data NestedList a = Elem a | List [NestedList a]   flatten :: NestedList a -> [a] flatten (Elem x) = [x] flatten (List x) = concatMap flatten x
  • compress :: Eq a => [a] -> [a] compress = map head . group We simply group equal values together (group), then take the head of each. Note that (with GHC) we must give an explicit type to compress otherwise we get:
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