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What does "lazy pattern match" mean and what is the meaning of the tilde in pattern matches?

with [[lazy pattern matching]] or with strict pattern matching? With the first definitions with lazy matching the laws are violated:

| lazy | pattern matching

That's the proof that the implementation is lazy. == First attempt - quadratic runtime, not lazy ==

Haskell is often described as a lazy language. ...ntics|non-strict]], which is not quite the same thing as [[lazy evaluation|lazy]].

...l, mainly because of its purity, lazyness, presence of lazy lists, pattern matching. Martín Escardó has many exact real arithetic materials also among his ma ...ble Technique in the World to compute pi] describes how to compute Pi as a lazy list of digits.

...only evaluated when needed, and <code>(:)</code>, a.k.a. ''"cons"'', is a lazy data constructor. The machinery used to implement this is called a thunk. I ...they consume only enough value to produce the value. As an example of how lazy functions work, consider:

...inct from <hask>Any ⊥ :: Any</hask>. What this means is that the following pattern match: must evaluate its argument, even though it seems like the pattern match can't fail: we must check whether <hask>x</hask> is <hask>⊥</hask>

;[http://video.google.com/videoplay?docid=8540922190337591178 SmallCheck and Lazy SmallCheck: automatic exhaustive testing for small values] ...rns, But Enough - an automatic verifier for partial but sufficient pattern matching]

...attern-matching against non-free data types, directly representing pattern-matching against a wide range of data types such as lists, multisets, sets, trees an ...hat offers pattern matching, lambda calculus consistency, macros, optional lazy evaluation, static type checking, one of the most powerful systems for typi

=== Tip: Haskellers love pattern matching === === Tip: pattern matching and guards can be mixed and matched ===

...//enfranchisedmind.com/blog/archive/2007/07/10/279 The "ole in the middle "pattern] * [http://www.aboutcode.net/2007/09/27/Lazy+Functions+In+LINQ.aspx Lazy functions in C#]

...onsume values from a (finite or infinite) supply. Note that due to pattern matching, running out of supply in a non-MonadZero monad will cause an error. == Lazy and Strict variants ==

...ell's main features is [[non-strict semantics]], which is implemented by [[lazy evaluation]] in all popular Haskell compilers. In this article we want to go through some techniques on how to check lazy behaviour on functions, examples of typical constructs which break laziness

pattern matching stack whose entries are essentially case ...you are getting stack overflows. However, making code tail-recursive in a lazy language is not quite the same as in a eager language. This page is more ge

...ages and vice versa, due to strong typing, automatic memory management and lazy evaluation. ...the rest of this resource first by Haskell construct (data types, pattern matching, integers), and then within each category to describe techniques that apply

...Hood - A Comparative Evaluation of Three Systems for Tracing and Debugging Lazy Functional Programs] ;[ftp://ftp.ida.liu.se/pub/labs/pelab/papers/aadebug93ladb.ps.gz Lazy algorithmic debugging: ideas for practical implementation.]

Haskell is a [[lazy]] language. It never performs a computation unless it needs to. This is not In Haskell, a string is a list of characters. Because Haskell is lazy, elements of a list are only evaluated when their contents are required for

to illustrate the difference between strict versus lazy constructor * In [[pattern guard]]s, "matches":

...compiler will automatically infer (= figure out) one for you if you're too lazy to write down a type signature yourself. Of course, parenthesis are allowed ::Exponentiation again, this time with ''pattern matching''. The first equation that matches will be chosen.

! Lazy? ...rt String, (Seq Char), ByteString, and (except for regex-posix) ByteString.Lazy.

You must keep in mind that due to lazy evaluation your traces will only print if the value they wrap is ever deman ...ell.org/~ndm/safe/ the safe library]. If you get an error message such as "pattern match failure, head []", you can then use <tt>headNote "extra information"<

* Is a lazy, purely functional language (that also has imperative features such as side * [[Lazy evaluation]]: results are only computed if they're required (strictness opt

metaprogramming, persistency, functional programming, lazy evaluation, pattern matching, terse syntax for anonymous lambdas, list

One way of seeing this might be to consider Haskell as a pattern language: I think many, if not most, design patterns commonly found in design pattern catalogs

...psu.edu/viewdoc/download?doi=10.1.1.53.3729&rep=rep1&type=pdf Implementing lazy functional languages on stock hardware: the Spineless Tagless G-machine], S '''!''' for '''`seq`''' is reminiscent of the new bang-pattern proposal for

...solution you can simply extract the first element of the resulting list - lazy evaluation will make sure that nothing is needlessly computed. ...see that we have two definitions of the functions. This is called pattern matching and we can briefly say that it will test the argument passed to the functio

* use the same simple regex pattern match-replace to remove FASTA sequence descriptions and all wildcard in one position), and (one pattern at a time) count matches in the redirected file

...tp://citeseer.ist.psu.edu/launchbury93natural.html A Natural Semantics for Lazy Evaluation] ...c.uk/oucl/work/jeremy.gibbons/publications/typecase.pdf TypeCase: a design pattern for type-indexed functions]

...ymorphism|polymorphically]] [[typing|statically typed]], [[Lazy evaluation|lazy]], [[functional programming|purely functional]] '' language, ...empty list on its left-hand side, and the second, with <code>(p:xs)</code> pattern on its left-hand side standing for non-empty list with the head element <co

### do has some other "goodies" like pattern matching and "lets" lets desugar to the obvious thing. ...he real world! in reality IO enforces ordering, which lets haskell do its lazy evaluation. It doesnt really represent "the real world" internally some ho

...he first few times you do it, but it works fine - remember, Haskell is a ''lazy'' language. This means that while you are building the node, you can set th ...l/Knuth-Morris-Pratt-in-Haskell Knuth-Morris-Pratt algorithm for substring matching]

...ibrary, based on runProcess in the standardised posix library. It provides lazy output from subprocesses. ...library for working with files and directories. Includes code for pattern matching, finding files, modifying file contents, and more.

they lack lazy evaluation, Guards are extended to [[pattern guard]]s and

"Sample: Pattern match failure in function at 9:1-9:15." match it against each pattern in turn, and finally reduce the body of

===Part V: Making Sense of 'fibs', and Why Lazy Evaluation is Important=== ...why does the list only get evaluated as far as you print it? Haskell is 'lazy', meaning that it doesn't do work that it doesn't have to. C programmers k

<li><p><em>Lazy SmallCheck 0.1</em>. Matthew Naylor ...[http://article.gmane.org/gmane.comp.lang.haskell.general/15702 announced] Lazy SmallCheck 0.1, a library for exhaustive, demand-driven testing of Haskell

Use pattern-matching. If <hask>mx :: Maybe Int</hask>: If you pattern-match without a <hask>Nothing</hask> case:

...onadic type <code>m a</code>. The expression to the left of the arrow is a pattern to be matched against the value ''inside'' the monad. <code>(x:xs)</code> w ...o standard monadic operators is roughly that every expression matched to a pattern, <code>x <- expr1</code>, becomes

...easy to use operation; full support of text and binary transfers; optional lazy interaction; server can serve up a real or a virtual filesystem tree; Stand ...blem with pattern match errors, and the only helpful message they get is: 'pattern match error'. The Safe library hopes to eliminate some of the frustration t

...kage.haskell.org/packages/archive/mtl/latest/doc/html/Control-Monad-Writer-Lazy.html <code>Control.Monad.Writer</code>] module provides the <code>Writer</c ...ckage.haskell.org/packages/archive/mtl/latest/doc/html/Control-Monad-State-Lazy.html <code>Control.Monad.State</code>] module provides the <code>State s a<