Question

I am puzzled as to why the 3rd function would not work :

   let generate1 = id
   let generate2 = let a = 1
                   id
   let generate3 = printfn "hi"
                   id

while the first 2 are fine, the last one spits out

error FS0030: Value restriction. The value 'generate3' has been inferred to have generic type
    val generate3 : ('_a -> '_a)    
Either make the arguments to 'generate3' explicit or, if you do not intend for it to be generic, add a type annotation.

Answer 1

I won’t attempt to explain value restriction, but I will attempt to sort out the semantic differences between these three values.

generate1 is just an alias for id, so we’re good there.

generate3 does some computations before returning id, hitting value restriction.

Then why doesn’t generate2 hit value restriction like generate3? Because the compiler can see that let x = 1 in id is semantically equivalent to just id: 1 is a constant expression and x not used in the body of the let ... in ... expression, so the compiler can and does throw them away. On-the-other-hand, if you replace 1 with a potential side-effect like sin 2.3 (sin is pure, but the compiler can’t prove it), then the compiler can’t safely reduce the expression, thus hitting value restriction as with generate3.