Records With Non Unique Field Names

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Summary

F# supports records in the same file having overlapping field names. This raises some issues.

F# record types are simple named types that support pattern matching and

construction using a field-assignment syntax:

 type MyRecord = { field1: int; field2: string }
 let myRecordValue = { field1 = 3; field2 = "3" }

F# supports records in the same file having identical or overlapping sets of

field names. For example:

 type MyRecord1 = { field1: int; field2: string }
 type MyRecord2 = { field2: string; field3: int }

Ambiguity on access is resolved through type annotations via the same mechanism as that used for resolving .NET method and field names, i.e. the type of the expression must be known using the information available earlier in the file, where this information is processed left-to-right according to the standard Hindley-Milner inference algorithm. Sometimes an ExplicitTypeAnnotation will be required.

Question

What does "Sometimes an ExplicitTypeAnnotation will be required." mean? I would assume that it mean placing the type name after the indentifer, but in the below sample only thing3 compiles. It seems that if you do not use the ObjectExpressions syntax only the type define last will compile.

Answer

(Don Syme, 20/4/06): Above "annotation" was referring to the annotations required to disambiguate accessed to fields of records, rather than creating record values. You are correct that if you wish to disambiguate record constructions you have to use the object-expression syntax, otherwise all field names must refer to the most recent record type in scope using any of the field names. We will consider type-inference-directed resolution for a future release.

        type MyRecord1 = { field1: int; field2: string }
        type MyRecord2 = { field2: string; field3: int }


        let thing1 = { field1 = 1 ; field2 = "" }
        let (thing2 : MyRecord1) = { field1 = 1 ; field2 = "" }
        let thing3 = { field2 = "" ; field3 = 1 }

As of version 1.1 it is possible to resolve ambiguity on creation of records.

The syntax follows that of ObjectExpressions, which use "and" to separate the items. For example:

 { new MyRecord2 with field2 = 3 and field3 = "3" }  
 { new MyRecord1 with field1 = 2 and field3 = 4 }
add a one or paragraph summary or description of what's discussed here;  put yours after 'Summary:'8/25/2005 6:02:07 AM - -81.146.39.71
add a one or paragraph summary or description of what's discussed here;  put yours after 'Summary:'8/25/2005 6:02:07 AM - -81.146.39.71

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