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Sample Port of F# Uservoice Suggestions
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Make tuple defined DU cases more consistent with tuple #65

Open cloudRoutine opened 7 years ago

cloudRoutine commented 7 years ago

Make tuple defined DU cases more consistent with tuple [12820011]

Submitted by Gauthier Segay on 3/5/2016 12:00:00 AM
[ 9 votes ]

It is possible to pattern match tuples in a flexible way: let a = 1,2 match a with | 1, -> () | -> () but it's not possible to do this on tuple defined DU cases, one has to write this: match pat with | SynPat.LongIdent(, , , , , ) -> () | ... which breaks whenever the case definition change (even at place where we don't care of the contents) while supporting this would be friendlier / more resilient to DU case change: match pat with | SynPat.LongIdent _ -> () or even match pat with | SynPat.LongIdent foo -> () // can deconstruct foo like a normal tuple

cloudRoutine commented 7 years ago

Posted by Gauthier Segay on 3/5/2016 2:38:00 PM

Actually found out it works with enclosing parens: match pat with | SynPat.LongIdent(_) -> () but it doesn't work with identifier (case whose utility is close to 0) match pat with | SynPat.LongIdent(a) -> ()

cloudRoutine commented 7 years ago

Posted by Alexei Odeychuk on 3/5/2016 3:02:00 PM

I support the idea shared by Gauthier Segay in respect of "match pat with | SynPat.LongIdent foo -> ..."! When I developed a natural language processing application in F#, I often wrote something like: match arabicLexeme with | Participle(, , , , , , ) -> ... | FiniteVerb(, , , , , , , , , ) -> ... Right now, the F# 4.0 compiler emits an error FS0019: This constructor is applied to 1 argument(s) but expects 6 in response to a line of code like "match pat with SynPat.LongIdent foo -> ...". There is no doubt, syntax for discriminated union cases is worth to be improved in this context. P.S. As to "match pat with SynPat.LongIdent ->" I tested it and found that the compiler (F# 4.0, Visual Studio 2015, Windows 7) accepted it. My test code for the F# Interactive was: type SynPat = | LongIdent of int * int * int * int * int * int let x : SynPat = LongIdent(1, 2, 3, 4, 5, 6) let y = match x with LongIdent _ -> true The F# Interactive response was: type SynPat = | LongIdent of int * int * int * int * int * int val x : SynPat = LongIdent (1,2,3,4,5,6) val y : bool = true No warnings or errors from the F# 4.0 compiler. P.P.S. When I tried to write in the F# Interactive: let y = match x with LongIdent foo -> true I received a compiler error FS0019: This constructor is applied to 1 argument(s) but expects 6.