Creating a partially applied function leads to error contains the non-generalizable type variable(s): '_weak1

Learning
1

Heya!

I’m using Dream together with Tyxml and I have this ugly function I’d like to move to a helper function to make it more readable

let string_of_html = Format.asprintf "%a" (Html.pp_elt ())

However, it runs into issues with the type variable being non-generalizable

error contains the non-generalizable type variable(s):'_weak.

When I use Format.asprintf "%a" (Html.pp_elt ()) in a context, it deduces the variable correctly, but I am wondering if there is a way to type string_of_html so I can use that instead?

2

Add a parameter:

let string_of_html x = Format.asprintf "%a" (Html.pp_elt ()) x
1 Like
3

Add a parameter:

let string_of_html x = Format.asprintf "%a" (Html.pp_elt ()) x

Ah it was that simple. I could’ve sworn I tried it haha. Thank you

4

To expand on @emillon’s answer, this is the infamous “value restriction”: let-bound values are only generalized (made polymorphic) if the right-hand side of the = is syntactically a “value” (eg a function). In particular, a function application is not one and so the definition is not generalized.

Eta-expansion (“adding a parameter”) can sometimes be used to work around this restriction, but one should keep in mind that the application in the right-hand side will now be executed each time that the bound variable is invoked, instead of only once when the variable is bound.

Cheers,
Nicolas

1 Like
5

I feel that this sentence is a bit too confusing the way it is worded. Indeed, eta-expanding does not work around the value restriction. On the contrary, it triggers the value restriction (just by definition), thus allowing the compiler to generalize the type.

6

Thanks for the comment. My understanding of the English phrase “trigger a restriction” is that of not satisfying the condition defining the restriction. In that sense, I interpret “work around the restriction” or “avoid the restriction” as indicating a way to satisfy the condition in question. So I would interpret “triggering the value restriction” to mean exactly that the program fails to typecheck, which is the opposite of how you interpreted it :slight_smile: I may be wrong, clarification is welcome!

Cheers,
Nicolas

7

I would say “satisfy the restriction”: an eta expansion is a value so its type can be generalized.