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Sorting Items by Computed Value - Schwartzian Transform?
f#

I have a list of objects, well actually method-augumented records. I need to order this list by value computed from the contents of each individual record.

I can figure out how to do a Schwartzian Transform (ST) - map to list of pre_computed_val * orig record, sort these by pre_computed_val, and then extract the records in order.

Before doing so, I want to check whether or not this is 'built-in' in F# as it is in Python these days.

I'm guessing not?

Specifically, I'm curious about List.sort_by ... obviously if any notion of ST was going to be built in, this is where it would likely live? ... but even more simple question first: the docs give nothing apart from the type signature. Am I to assume that List.sort_by accepts a function which transforms the data (but maybe 'on the fly' for every comparison and not ST?) and sorts with the default compare function? Not so great minds want to know!

I hope we can get a good answer here as Google currently gives nothing interesting for this search term (well, some stuff for Haskell).

.

Hello.

In F# sources List.sort_by is defined as wrapper other sort:

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let sort_by f l = sort (fun x y -> compare (f x) (f y)) l

so, you can use simple Perl's idea translation:

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List.map (fun el -> (el.Func(),el)) your_list 
  |> List.sort (fun (h1,v1) (h2,v2) -> compare h1 h2)
  |> List.map (fun (_,el) -> el)
By on 2/1/2009 3:07 AM ()

Very clear answer thanks!

It never occurred to me that I could actually peruse F# library source code to see the definition of List.sort_by (it actually does what I thought it might.. i.e. 'not enough' by itself).

By on 2/1/2009 5:39 AM ()

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