How to Resolve 'cannot infer V: infer type parameter from constraint implementation' in Go Generic Interfaces?

Error Resolving: "cannot infer V: infer type parameter from constraint implementation" in Go Generic Interfaces

In Go, defining a generic interface with type parameters can lead to inference issues when implementing and using it. This article addresses the error "cannot infer V: infer type parameter from constraint implementation" in such scenarios.

Problem Description

Consider an interface ConfigStorage with type parameters K and V, where V extends WritableType. A file system storage implementation of this interface, FileSystemStorage, is defined. However, when attempting to use the generic function GetValue, a compilation error arises due to inability to infer the type of V.

Solution

Go 1.21 and Higher:

The issue is resolved in Go 1.21 and later as type inference has been enhanced to consider method signatures in interfaces. Type arguments for type parameters in method signatures can be inferred from matching parameters in the corresponding methods.

To use GetValue without specifying type constraints:

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

Go 1.20 and Lower:

For earlier Go versions, inference of V from the type implementing the constraint is not supported. Explicit type parameters must be provided when calling GetValue:

GetValue[string, string](fileStorage, "key")

Explanation

The error arises because the function GetValue attempts to infer the type of V from the provided arguments: its first argument, storage, and its second argument, key. However, these arguments alone do not provide sufficient information to determine V because storage is an interface value that may implement multiple types.

Explicitly specifying the type parameters eliminates the ambiguity for the compiler. It allows the type inference algorithm to deduce the correct type of V based on the provided arguments.

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