How Can I Resolve Go's 'cannot infer V' Error When Implementing Generic Interfaces?

Inferring Type Parameters for Generic Interface Implementations: "cannot infer V" Conundrum

Issue

When attempting to save and load results of varying types using a generic interface and its implementation, Go encounters an issue inferring the V type:

cannot infer V: infer type parameter from constraint implementation

Explanation

In Go 1.20 and earlier, inferring the type V from the implementation of the generic constraint ConfigStorage[K, V] is not straightforward. The type inference algorithm is limited in its ability to deduce type arguments from concrete types implementing constraints.

Solution

Go 1.21 and Above:

Since Go 1.21, this issue is resolved. Type inference now considers types used in interface methods. Therefore, you can simply call GetValue without specifying type constraints:

result, _ = GetValue(fileStorage, "key")

Go 1.20 and Below:

In older versions of Go, you must explicitly specify the type parameters for GetValue:

GetValue[string, string](fileStorage, "key") // First string for K, second for V

Additional Information

The Go release notes highlight that type inference now considers methods when assigning values to an interface. This allows type arguments for type parameters in method signatures to be inferred from matching parameter types of methods.

Prior to Go 1.21, proposals suggested using function argument type inference to deduce type arguments from non-type arguments. However, constraint type inference was not initially supported for deducing unknown type arguments from known ones. This limitation led to the "cannot infer V" error when inferring V from the type implementing the constraint.

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