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Error message not properly located (or phrased) with or-pattern #6308

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vicuna opened this Issue Jan 27, 2014 · 4 comments

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vicuna commented Jan 27, 2014

Original bug ID: 6308
Reporter: @alainfrisch
Assigned to: @alainfrisch
Status: closed (set by @xavierleroy on 2015-12-11T18:25:52Z)
Resolution: fixed
Priority: normal
Severity: minor
Fixed in version: 4.02.0+dev
Category: typing
Monitored by: @gasche

Bug description

Consider the following error message:

type 'a t = {x: 'a}
function {x=a} | {x=(a,_)} -> ();;
         ^^^^^^^^^^^^^^^^^
Error: This pattern matches values of type 'a * 'b
       but a pattern was expected which matches values of type 'a
       The type variable 'a occurs inside 'a * 'b

The location of the error is the whole or-pattern, which is not coherent with its text.

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vicuna commented Jan 27, 2014

Comment author: @alainfrisch

It all happens in the enter_orpat_variables function (in typecore.ml). One must decide between using l2 instead of loc for the Pattern_type_clash error (and calling "unify env t2 t1" instead of "unify env t1 t2"), or using a different error which clearly indicates that the problem is related to the or-pattern (and gives the variable name).

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vicuna commented Jan 27, 2014

Comment author: @alainfrisch

I've committed a fix using a different error message, commit 14421 on trunk. If anyone want to argue in favor of keeping the old error with a different location (or suggest a better wording of the new message), don't hesitate!

# type 'a t = {x: 'a};;
type 'a t = { x : 'a; }
# function {x=Some a} | {x=a} -> ();;
           ^^^^^^^^^^^^^^^^^^
Error: The variable a on the left-hand side of this or-pattern has type 'a
       but on the right-hand side it has type 'a option
       The type variable 'a occurs inside 'a option
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vicuna commented Jan 27, 2014

Comment author: @lpw25

Does that message still make sense if there are more than two branches to the or-pattern?

For example:

type 'a t = {y : bool; x: 'a};;

function {y = true; _} | {x=Some a; _} | {x=a; _} -> ();;
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vicuna commented Jan 28, 2014

Comment author: @alainfrisch

function {y = true; _} | {x=Some a; _} | {x=a; _} -> ();;

Error: Variable a must occur on both sides of this | pattern

If you meant:

# function {y = a; _} | {x=Some a; _} | {x=a; _} -> ();; 
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error: The variable a on the left-hand side of this or-pattern has type 
       bool but on the right-hand side it has type bool option
# type t = { x : int; y : string; z : bool; };;
# function {x = a; _} | {y = a; _} | {z = a; _} -> ();;
           ^^^^^^^^^^^^^^^^^^^^^^^
Error: The variable a on the left-hand side of this or-pattern has type 
       int but on the right-hand side it has type string

This seems decent to me.

@vicuna vicuna closed this Dec 11, 2015

@vicuna vicuna added the typing label Mar 14, 2019

@vicuna vicuna added the bug label Mar 20, 2019

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