less - Restrict mixin values and throw exception when invalid value is provided -

i have following less mixin:

.box-sizing(@value) {     -webkit-box-sizing: @value;     -moz-box-sizing: @value;     box-sizing: @value; } 

and allow values 'border-box' , 'content-box' parameter, otherwise less engine should throw exception. how can achieve this? because without validation can write value mixin , work, generate invalid css , no 1 notice there error.

as far know, there no way make less compiler throw error invalid value described in question. however, can make less compiler not produce output @ when invalid value provided making use of guards feature.

in below snippet, mixin invoked when either of 2 valid values passed input. if different value provided, less compiler find no match , hence wouldn't output anything.

.box-sizing(@value){     & when (@value=content-box) , (@value=border-box){ /* comma or operator */         -webkit-box-sizing: @value;         -moz-box-sizing: @value;         box-sizing: @value;     } } #demo{     .box-sizing(content-box); } 

less has built-in type functions can used along guards check if value valid or not (like if input number or color etc). there iskeyword function none of these check invalid css value.

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if have wide list of valid values make use of loops feature below. here, extract each value array of valid values , if input value matches 1 of them, output properties. if input not match input value, nothing output (again).

@valid-values: content-box, border-box, padding-box; .box-sizing(@value){     .loop-valid-values(@index) when (@index > 0){         @valid-value: extract(@valid-values, @index);         & when (@value= @valid-value){             -webkit-box-sizing: @value;             -moz-box-sizing: @value;             box-sizing: @value;         }         .loop-valid-values(@index - 1);     }     .loop-valid-values(length(@valid-values)); } #demo{     .box-sizing(content-box); } 

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strictly not recommended if insist on making compiler throw exception or other when invalid value provided deliberately introduce error in not-valid-value part.

.box-sizing(@value){     & when (@value= content-box), (@value= border-box){         -webkit-box-sizing: @value;         -moz-box-sizing: @value;         box-sizing: @value;     }     & when not (@value= content-box), (@value= border-box){         output: @bwahahaha; /* there no such variable , hence when input value not valid, compiler complain variable undefined */     } } #demo{     .box-sizing(conten-box); }