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Top Level (Interactive) vs static compilation question
f#compilercollections

The following definition compiles and runs when I compile and build my F# class but when I try to export this definition to the top level I get the following error:

Definition:

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let omDestinationTable = Hashtbl.create 30

error: FS0030: Value restriction. Type inference has inferred the signature val omDestinationTable :HashMultiMap<'_a, '_b>

Either define 'omDestinationTable' as a simple data term, make it a function, or add a type constraint to instantiate the type parameters.

Now is this allowed during regular compilation because the compiler can infer the type based on my use of the table?

What is the proper way to define a Hashtble?

Should I just move on to HashMultiMap?

I like the way the immutable Map type works better, does it offer constant access times?

I need some collections guidance

Thanks,

Parnell

.

The code is valid with compilation, because type inference can use the whole file to infer the exact type. In interactive mode, type inference can only use the current phrase (and old ones).

First solution (the two lines are sent in the same time):

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> let omDestinationTable = Hashtbl.create 30
let _ = Hashtbl.add omDestinationTable "foo" 2;;
val omDestinationTable : HashMultiMap<string,int>

Second solution (add type annotations):

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let omDestinationTable: (int, string) Hashtbl.t = Hashtbl.create 30;;

or:

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let omDestinationTable: HashMultiMap<string,int> = Hashtbl.create 30;;

Laurent.

By on 5/30/2007 9:18 AM ()

By the way HashMultiMap (the .NET type) and HashTable (this comes from OCaml) are the same types. From hashtbl.fs:

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type HashTable<'key,'a> = Microsoft.FSharp.Collections.HashMultiMap<'key,'a>


type ('key,'a) t = Microsoft.FSharp.Collections.HashMultiMap<'key,'a>

I suggest you to use HashTable. It allows you to call .NET methods as well as functions defined in the fslib.

Unfortunately, Maps don't offer constant access times, because it's implemented using a binary tree (at least, this is how it's defined in OCaml, I believe it's similar in .NET). So most operations are logarithmic (in the size of the map). If performances are very important, you can make some tests. If your map isn't huge, it may be as fast as the hashtable.

I usually prefer maps, because there are no side-effects.

Laurent.

By on 5/30/2007 9:49 AM ()

Thanks for the information, from my initial tests Map seems to be as fast as hashtable, building it is actually faster.

Parnell

By on 5/30/2007 11:42 AM ()

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