Saturday, November 21, 2009

F# under the covers X -- the curious case of record types

This is unchanged in the February 2010 CTP (1.9.9.9) release.

They're coming thick and fast now...

Define a record type such as

The class that results looks like

where the source context for each method is the same -- the range containing just the type name.

Now, you wouldn't expect anything to be seriously unusual about this class in terms of its implementation, but there is.

FxCop reminds you about the IComparable should-haves that can't be enforced through interface constraints:

[Location not stored in Pdb] : warning : CA1036 : Microsoft.Design : 'Context' should define operator '!=' since it implements IComparable.
[Location not stored in Pdb] : warning : CA1036 : Microsoft.Design : 'Context' should define operator '<' since it implements IComparable.
[Location not stored in Pdb] : warning : CA1036 : Microsoft.Design : 'Context' should define operator '==' since it implements IComparable.
[Location not stored in Pdb] : warning : CA1036 : Microsoft.Design : 'Context' should define operator '>' since it implements IComparable.

but which aren't there; and Reflector's decompilation to C# is stymied by the highlighted CompareTo overloads -- the simple one is implemented as

which fortunately doesn't give much scope for things to go wrong.

In this and the previous case, the offending instruction that Reflector balks at is a simple branch such as indicated in this example

It's quite clear at every turn that F# is coming at the problem of code generation from a very different direction to the well explored parts of the phase space of valid IL that C# and VB.net dabble in. And clear, too, that this will present a significant challenge to all writers of tools to work with the language -- it takes us well out of our old comfort zone.

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