In the following code:
let lift = Ok
let a = Ok 1
let b = lift 1
In the 3rd line I'm getting error:
Value restriction. The value 'b' has been inferred to have generic type val b: Result<int,'_a>
Either define 'b' as a simple data term, make it a function with explicit arguments or, if you do not intend for it to be generic, add a type annotation.
Line 2 let a = Ok 1 works just fine, but line 3, where I'm using my helper function lift causes the error. Why is that? Is it a bug? It is F# 9.0. Below is a full output from fsi, which doesn't explain it to me.
> let lift = Ok;;
val lift: ResultValue: 'a -> Result<'a,'b>
> let a = Ok 1;;
val a: Result<int,'a>
> let b = lift 1;;
let b = lift 1;;
----^
stdin(11,5): error FS0030: Value restriction: The value 'b' has an inferred generic type
val b: Result<int,'_a>
However, values cannot have generic type variables like '_a in "let x: '_a". You can do one of the following:
- Define it as a simple data term like an integer literal, a string literal or a union case like "let x = 1"
- Add an explicit type annotation like "let x : int"
- Use the value as a non-generic type in later code for type inference like "do x"
or if you still want type-dependent results, you can define 'b' as a function instead by doing either:
- Add a unit parameter like "let x()"
- Write explicit type parameters like "let x<'a>".
This error is because a let binding without parameters defines a value, not a function. Values cannot be generic because reading a value is assumed to result in the same everywhere but generic type parameters may invalidate this assumption by enabling type-dependent results.
In the following code:
let lift = Ok
let a = Ok 1
let b = lift 1
In the 3rd line I'm getting error:
Value restriction. The value 'b' has been inferred to have generic type val b: Result<int,'_a>
Either define 'b' as a simple data term, make it a function with explicit arguments or, if you do not intend for it to be generic, add a type annotation.
Line 2 let a = Ok 1 works just fine, but line 3, where I'm using my helper function lift causes the error. Why is that? Is it a bug? It is F# 9.0. Below is a full output from fsi, which doesn't explain it to me.
> let lift = Ok;;
val lift: ResultValue: 'a -> Result<'a,'b>
> let a = Ok 1;;
val a: Result<int,'a>
> let b = lift 1;;
let b = lift 1;;
----^
stdin(11,5): error FS0030: Value restriction: The value 'b' has an inferred generic type
val b: Result<int,'_a>
However, values cannot have generic type variables like '_a in "let x: '_a". You can do one of the following:
- Define it as a simple data term like an integer literal, a string literal or a union case like "let x = 1"
- Add an explicit type annotation like "let x : int"
- Use the value as a non-generic type in later code for type inference like "do x"
or if you still want type-dependent results, you can define 'b' as a function instead by doing either:
- Add a unit parameter like "let x()"
- Write explicit type parameters like "let x<'a>".
This error is because a let binding without parameters defines a value, not a function. Values cannot be generic because reading a value is assumed to result in the same everywhere but generic type parameters may invalidate this assumption by enabling type-dependent results.
The compiler performs automatic generalization only on complete function definitions that have explicit arguments, and on simple immutable values. This means that the compiler issues an error if you try to compile code that is not sufficiently constrained to be a specific type, but is also not generalizable.
Ok is a union case, so Ok 1 is considered a simple immutable value, and the compiler will generalize its type to Result<int, 'a> in the absence of an explicit error type.
However, lift is a function, not a union case. (F# allows you to create a function from a union case without further ceremony.) Since lift 1 does not specify an error type, the value restriction is triggered.