[Haskell-cafe] Why I Love Haskell In One Simple Example

[Claus Reinke]

> 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

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