Haskell: Converting Int to String
The opposite of read is show. Prelude> show 3 “3” Prelude> read $ show 3 :: Int 3
haskell
Comparing Haskell’s Snap and Yesod web frameworks
Full disclosure: I’m one of the lead developers of Snap. First of all, let’s talk about what Snap is. Right now the Snap team maintains five different projects on hackage: snap-core, snap-server, heist, snap, and xmlhtml. snap-server is a web server that exposes the API defined by snap-core. heist is a templating system. xmlhtml is … Read more
Haskell: Lists, Arrays, Vectors, Sequences
Lists Rock By far the most friendly data structure for sequential data in Haskell is the List data [a] = a:[a] | [] Lists give you ϴ(1) cons and pattern matching. The standard library, and for that matter the prelude, is full of useful list functions that should litter your code (foldr,map,filter). Lists are persistant … Read more
Getting associated type synonyms with template Haskell
It is not implemented because nobody requested it. The odd thing is that TH uses its own AST, which doesn’t follow internal compiler’s AST. As a result, any new feature (e.g. associated type families) is not automatically available via TH. Some one have to open a ticket and implement it. For the reference: internal reifyClass … Read more
How fundamentally different are push-pull and arrowized FRP?
I took a trip to Haskell.org to investigate your question What I found are two important papers you ought to read to further your research, and I am building my answer to your question from these scholarly papers. Push-Pull FRP by Conal Elliott Generalising Monads to Arrows by John Hughes Yes, but also no. According … Read more
What’s so bad about Template Haskell?
One reason for avoiding Template Haskell is that it as a whole isn’t type-safe, at all, thus going against much of “the spirit of Haskell.” Here are some examples of this: You have no control over what kind of Haskell AST a piece of TH code will generate, beyond where it will appear; you can … Read more
What does the exclamation mark mean in a Haskell declaration?
It’s a strictness declaration. Basically, it means that it must be evaluated to what’s called “weak head normal form” when the data structure value is created. Let’s look at an example, so that we can see just what this means: data Foo = Foo Int Int !Int !(Maybe Int) f = Foo (2+2) (3+3) (4+4) … Read more
Abusing the algebra of algebraic data types – why does this work?
Disclaimer: A lot of this doesn’t really work quite right when you account for ⊥, so I’m going to blatantly disregard that for the sake of simplicity. A few initial points: Note that “union” is probably not the best term for A+B here–that’s specifically a disjoint union of the two types, because the two sides … Read more
What is Weak Head Normal Form?
I’ll try to give an explanation in simple terms. As others have pointed out, head normal form does not apply to Haskell, so I will not consider it here. Normal form An expression in normal form is fully evaluated, and no sub-expression could be evaluated any further (i.e. it contains no un-evaluated thunks). These expressions … Read more
Why is Haskell (GHC) so darn fast?
I agree with Dietrich Epp: it’s a combination of several things that make GHC fast. First and foremost, Haskell is very high-level. This enables the compiler to perform aggressive optimisations without breaking your code. Think about SQL. Now, when I write a SELECT statement, it might look like an imperative loop, but it isn’t. It … Read more