Why can't F#'s type inference handle this?

Type inference works left-to-right. This is where the pipeline operator is useful; if you already know the type of 'fis', then write it as fis |> Seq. Map (fun fi -> fi.Name) and the inference works for you (In general, expressions of the form o.

Property o. Method args require the type of 'o' to be known a priori for most other expressions, when a type is not pinned down the inference system can 'float a constraint' along that can be solved later, but for these cases, there are no constraints of the form 'all types with a property named P' or 'all types with a method named M' (like duck typing) that can be postponed and solved later.So you need that info now, or inference fails immediately. ) See also an overview of type inference in F.

Type inference works left-to-right. This is where the pipeline operator is useful; if you already know the type of 'fis', then write it as fis |> Seq. Map (fun fi -> fi.Name) and the inference works for you.

(In general, expressions of the form o. Property o. Method args require the type of 'o' to be known a priori; for most other expressions, when a type is not pinned down the inference system can 'float a constraint' along that can be solved later, but for these cases, there are no constraints of the form 'all types with a property named P' or 'all types with a method named M' (like duck typing) that can be postponed and solved later.

So you need that info now, or inference fails immediately. ) See also an overview of type inference in F#.

Great clarification. +1 – Jose Basilio May 10 '09 at 5:29 3 I hope they make the type inference more robust. I've already hit a few cases where I had to reorder the methods in a class or I would get too generic type errors.

– gradbot May 10 '09 at 14:49 1 For what its worth, this blog post and some of its comments have a useful explanation of some of F#'s strengths and weaknesses in its type checker: neilmitchell.blogspot. Com/2008/12/… – Juliet May 10 '09 at 19:14.

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