Re: [Haskell-cafe] Why I Love Haskell In One Simple Example
Hi Mads, Since ghc-6.4 there's another feature that is enabled by such explicit foralls in type signatures, namely scoped type variables. Consider foo :: Num a = a - a - a foo x y = x + z where z = 2 * y Now since adding type signatures is a good thing, you want to give z an explicit type signature. But |z :: a| fails with an Inferred type is less polymorphic than expected error because the |a| actually means |forall a. a| and not the |a| from foo's type signature. The classical way (still, it's an extension implemented in hugs and all versions of ghc i'm aware of) to bring the in scope by binding it like this: foo (x :: a) y = x + y where This is fine in such simple examples, but often gets tedious and clutters up the definition by weird type annotations. Therefore ghc-6.4 implements the great feature that the |a| from foo's type signature is automatically brought into scope if you explicitely quantify it with a forall. Aside: A small disadvantage seems to be that you can only scope over either all or none of the type variables in your signature. However, foo :: forall a. Num a = (forall b. ...) will bring the variable a into scope, but not b and is otherwise equivalent to foo :: forall a b. Num a = ... On 6/27/05, Mads Lindstrøm [EMAIL PROTECTED] wrote: snip I had newer seen anybody use forall a. in function signatures before, and therefore was curious about its effect. This is probably do to my inexperience regarding Haskell. However, I tried to remove it and wrote this instead: test :: (Num a) = a and the code still compiled and seems to run fine. Also using the prettyShow and rpnShow functions. So, why are you using the forall keyword? (this is not meant as a critique, i am just curious) I tried to find documentation about the use of the forall keyword in respect to functions (I do know about it in with respect to existentially quantified types), but with no luck. So, if anybody has some good pointers, please let med know about it. A great recource is http://haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html Bookmark this page and come back to it every once in a while - there are just so many treasures in it - one of my favorites is 7.4.12. Generalised derived instances for newtypes Thomas ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Why I Love Haskell In One Simple Example
I posted this on my blog at http://changelog.complete.org/node/339 but I thought there may be some here that would find it of use. I recently implemented some new Haskell numeric types that, instead of performing calculations, can generate a rendering of the requested calculation or store units with it. Here you see a transcript of my session with a Haskell interpreter. The mathematical statements I am entering after the are standard Haskell expressions, and, as I demonstrate, normally evaluate to a single result. Once I get a more powerful simplifier, I will probably write a LaTeX exporting function as well. The entire implementation of this, BTW, is less than 200 lines. NumTest 5 + 1 * 3 8 NumTest prettyShow $ 5 + 1 * 3 5+(1*3) NumTest rpnShow $ 5 + 1 * 3 5 1 3 * + NumTest prettyShow $ 5 + 1 * 3 5+(1*3) NumTest prettyShow $ simplify $ 5 + 1 * 3 5+3 NumTest prettyShow $ 5 * (Symbol x) + 3 (5*x)+3 NumTest 5 / 2 2.5 NumTest (units 5 m) / (units 2 s) 2.5_m/s NumTest (units 5 m) / 2 2.5_m NumTest 10 * (units 5 m) / (units 2 s) 25.0_m/s NumTest sin (pi/2) 1.0 NumTest sin (units (pi/2) rad) 1.0_1.0 NumTest sin (units 90 deg) 1.0_1.0 NumTest (units 50 m) * sin (units 90 deg) 50.0_m NumTest ((units 50 m) * sin (units 90 deg)) :: Units (SymbolicManip Double) 50.0*sin(((2.0*pi)*90.0)/360.0)_m NumTest rpnShow $ dropUnits $ ((units 50 m) * sin (units 90 deg)) 50.0 2.0 pi * 90.0 * 360.0 / sin * NumTest (units (Symbol x) m) * sin (units 90 deg) x*sin(((2.0*pi)*90.0)/360.0)_m Also, I defined this in my source file: test :: forall a. (Num a) = a test = 2 * 5 + 3 Now, it can be used: NumTest test 13 NumTest rpnShow test 2 5 * 3 + NumTest prettyShow test (2*5)+3 NumTest test + 5 18 NumTest prettyShow (test + 5) ((2*5)+3)+5 NumTest rpnShow $ test + 5 2 5 * 3 + 5 + You can grab the very early experimental code with darcs get http://darcs.complete.org/num. Haskell has no built-in support for numeric types with units, arbitrary symbols carried through computations, etc. But it was trivial to add it. This kind of extensibility is a key part of why Haskell is so amazing. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why I Love Haskell In One Simple Example
I recently implemented some new Haskell numeric types that, instead of performing calculations, can generate a rendering of the requested calculation or store units with it. good old Haskell rule (with apologies to Daniel Düsentrieb, I think;): impossible things are delayed immediately, miracles may take a little longer you can't get an expression representation from a value, but you can generate a representation while building the expression, so that you have it at hand if/when needed. I did something similar once (pairing values with representations instead of calculating values from representations), which you might find interesting for comparison (note that the focus in this one was on simplicity, eg. string representation only, no simplification, etc.): http://www.cs.kent.ac.uk/~cr3/toolbox/haskell/R.hs btw, that kind of thing gets even more interesting with ho-functions like foldr, foldl, map. try it!-) nice for explaining these hofs.. (the code still works, but if you use ghci -fglasgow-exts instead of hugs +98, the defaulting doesn't seem to kick in, so you'll need to give explicit type annotations, eg. map (+) [1..4::R Integer] or foldl (-) 0 [1..4::R Integer]). cheers, claus --- Do you have late-breaking research results in FP but have missed the other deadlines? Submit to TFP'05: http://www.tifp.org/tfp05/ --- extended abstracts: 8th July 2005 full papers: 2nd September 2005 symposium: 23/24 September 2005, Tallinn, Estonia (w.ICFP/GPCE) --- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
