.net - Trouble with a member constraint invocation expression when the member signature has type variables in F# -


i'm having trouble using member constraint invocation expressions when member signature has type variables. specifically, code

[<autoopen>] module foo  // preapplied lens type lens <'i,'j> = { : 'j; set : 'j -> 'i }  // record type lenses  type foo1 = {     num_ : int;     letter_ : char }     member this.num = {         = this.num_;         set = (fun num' -> {this num_=num'})}     member this.letter = {         = this.letter_;         set = (fun letter' -> {this letter_=letter'})} end  let (foo1:foo1) = {num_ = 1; letter_ = 'a'}  // ecord type lenses  type foo2 = {     num_ : int;     name_ : string }     member this.num = {         = this.num_;         set = (fun num' -> {this num_=num'})}     member this.name = {         = this.name_;         set = (fun name' -> {this name_=name'})} end  let (foo2:foo2) = {num_ = 2; name_ = "bob"}  // add 2 record num lens let inline add2 (x : ^t) =     let num = (^t : (member num : lens<^t,int>) (x)) in     num.get + 2

gives error:

test05.fsx(39,47): error fs0010: unexpected symbol ) in expression

the issue @ member constraint invocation expression on second last line. basically, can't figure out how pass type variables ^t , int lens type within member constraint invocation expression. there way accomplish this?

there must space between < , ^ in static constraint:

// add 2 record num lens let inline add2 (x : ^t) =     let num = (^t : (member num : lens< ^t,int>) (x)) in     num.get + 2

otherwise, both characters <^ treated name of infix function (aka "operator").


-

Popular posts from this blog

debugging - Reference - What does this error mean in PHP? -

what this? this number of answers warnings, errors , notices might encounter while programming php , have no clue how fix. community wiki, invited participate in adding , maintaining list. why this? questions "headers sent" or "calling member of non-object" pop on stack overflow. root cause of questions same. answers questions typically repeat them , show op line change in his/her particular case. these answers not add value site because apply op's particular code. other users having same error can not read solution out of because localized. sad, because once understood root cause, fixing error trivial. hence, list tries explain solution in general way apply. what should here? if question has been marked duplicate of this, please find error message below , apply fix code. answers contain further links investigate in case shouldn't clear general answer alone. if want contribute, please add "favorite" error message, warning or notic...
Read more

c++ - Why doesn't unordered_set provide an array access operator -

i'm curious why stl container unordered_set , has constant time complexity random access on average, not provide method accessing elements distance first element in container. example: t& unordered_set::operator[](size_t index) { return *(begin() + index); } accessing element "by distance" implies there meaningful way measure distance. trouble std::unordered_set is is, well, unordered . hence, there no meaningful way of explaining "some distance beginning" in non-arbitrary way. if want access distance, copy data vector: std::vector tmp(unordered.begin(), unordered.end());
Read more