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Why no implicit type conversion in F#

In C# the F# library doesn’t allow implicit type conversions:
Func.Invoke(“ab”) // compiler error

But if it could it would be used often. For example the F# implementation of the Seq interface is:
static member (++)[] (seqAsCompact: ‘SequenceAsCompact, elem: int) =
seqAsCompact.Cons (elem, seqAsCompact)

However from the fact that there is an error, one might thing that it is better to not allow it; but in fact it is because it provides explicit type conversions:
(seqAsCompact.Cons “foo” (int.Parse (Seq.head (Seq.ofSeq [1.0; 2.0; 3.0])))).ToString ()

Is the decision to not allow implicit type conversion a good or a bad idea?


In addition to the nice answers of the questions whether implicit type conversion is a good idea, a question if there is reason to keep it.
In the discussion about adding implicit conversions to the F# compiler, there is a note by Andy Wingo that mentions that implicit type conversions of the same type are automatically checked by the compiler and that this check allows a number of errors to be found before F# code is generated.
If you consider types with only one constructor, then implicit conversions of the same single constructor are the only possible implicit conversions an user can implicitly generate. That is very important for the automatic validation in the compiler, as mentioned by Andy Wingo, else all users that are aware of it can generate erroneous code.


F# would do that a lot if you could.
let f (x : int) = x + 1 // compile error

Because with one exception F# does not allow conversions “by definition” of function and operator it means that user can not use it in expressions, nor can they provide their own conversions.
And conversion to string is not one of exceptions – just compare with this:
let f (x : string) = x + 1 // no error, but what if the user

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