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typesocamlreasonbucklescript

ReasonML Signature Mismatch

I'm getting a signature mismatch in ReasonML between a type I defined which resolves to type mutationFunctionType = (~id: UUID.t, ~classroomId: UUID.t, unit) => unit; and 'a which I'm expecting. (seen below).

[1]   Signature mismatch:
[1]   ...
[1]   Values do not match:
[1]     let callMutationWithApollo:
[1]       ApolloMutation.apolloMutationType(Config.t) => mutationFunctionType
[1]   is not included in
[1]     let callMutationWithApollo:
[1]       ApolloMutation.apolloMutationType(Config.t) => 'a

I'm curious about why I'm receiving this error because I thought the polymorphic type `a could be treated as basically an any type.

Thanks

Solution

  • Type polymorphism allow you to write functions generically in some types. For instance the polymorphic identity has type 'a => 'a. It means it can be used at type int => int or bool => bool by instantiating 'a.

    However, this 'a does not mean anything goes. It only works in one direction: 'a can become anything, but anything cannot become 'a. So from ApolloMutation.apolloMutationType(Config.t) => 'a you can indeed get ApolloMutation.apolloMutationType(Config.t) => mutationFunctionType but what you're doing is the opposite.

    For instance you cannot turn an int => int function into a int => 'a function. By the way, providing such a function of type int => 'a or ApolloMutation.apolloMutationType(Config.t) => 'a is not really possible unless it returns errors or does not terminate. So I suggest updating your signature if possible to explicitly mention mutationFunctionType.