Doc: more details on the syntax of the selector and variable default operators
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@@ -122,7 +122,7 @@ _Expr_ supports three type of numbers:
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. [blue]#Integers#
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. [blue]#Floats#
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. [blue]#Factions# internally are stored as _pairs of_ Golang _int64_ values.
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. [blue]#Factions#
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In mixed operations involving integers, fractions and floats, automatic type promotion to the largest type take place.
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@@ -183,7 +183,7 @@ _dec-seq_ = _see-integer-literal-syntax_
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`>>>` [blue]`4.5E-3` +
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[green]`0.0045` +
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`>>>` [blue]`4.5E10` +
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[green]`4.5e+10` +
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[green]`4.5e+10`
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.Arithmetic operators
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@@ -207,7 +207,7 @@ _sign_ = "**+**" | "**-**" +
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_num-den-spec_ = _digit-seq_ "**|**" _digit-seq_ +
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_float-spec_ = _dec-seq_ "**.**" [_dec-seq_] "**(**" _dec-seq_ "**)**" +
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_dec-seq_ = _see-integer-literal-syntax_ +
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_digit-seq_ = _see-integer-literal-syntax_ +
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_digit-seq_ = _see-integer-literal-syntax_
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====
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.Examples
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@@ -241,7 +241,7 @@ Fractions can be used together with integers and floats in expressions.
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`>>>` [blue]`4 - 1|2` +
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[green]`7|2` +
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`>>>` [blue]`1.0 + 1|2` +
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[green]`1.5` +
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[green]`1.5`
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@@ -289,7 +289,8 @@ _item_ = _string-expr_ "**.**" _integer-expr_
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`>>>` [blue]`\#s` [gray]_number of chars_ +
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[gren]`3` +
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`>>>` [blue]`#"abc"` [gray]_number of chars_ +
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[green]`3` +
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[green]`3`
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=== Booleans
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Boolean data type has two values only: [blue]_true_ and [blue]_false_. Relational and boolean expressions result in boolean values.
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@@ -475,50 +476,65 @@ The assignment operator [blue]`=` is used to define variables in the evaluation
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The value on the left side of [blue]`=` must be an identifier. The value on the right side can be any expression and it becomes the result of the assignment operation.
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.Example
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[source,go]
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----
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a=15+1 // returns 16
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----
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`>>>` [blue]`a=15+1`
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[green]`16`
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=== Selector operator [blue]`? : ::`
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The _selector operator_ is very similar to the _switch/case/default_ statement available in many programming languages.
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.Syntax
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[source,bnf]
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----
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<selector-operator> ::= <select-expression> "?" <selector-case> { ":" <selector-case> } ["::" <default-multi-expression>]
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<selector-case> ::= [<match-list>] <case-value>
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<match-list> ::= "["<item>{","<items>}"]"
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<item> ::= <expression
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<case-multi-expression> ::= "{" <multi-expression> "}"
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<multi-expression> ::= <expression> {";" <expression>}
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----
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.Selector literal Syntax
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_selector-operator_ = _select-expression_ "*?*" _selector-case_ { "*:*" _selector-case_ } ["*::*" _default-multi-expression_] +
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_selector-case_ = [_match-list_] _case-value_ +
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_match-list_ = "*[*" _item_ {"*,*" _items_} "*]*" +
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_item_ = _expression_ +
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_case-multi-expression_ = "*{*" _multi-expression_ "*}*" +
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_multi-expression_ = _expression_ { "*;*" _expression_ } +
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_default-multi-expression_ = _multi-expression_
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In other words, the selector operator evaluates the expression (`<select-expression>`) on the left-hand side of the `?` symbol; it then compares the result obtained with the values listed in the `<match-list>`'s. If the comparision finds a match with a value in a match-list, the associated `<case-multi-expression>` is evaluted, and its result will be the final result of the selection operation.
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In other words, the selector operator evaluates the _select-expression_ on the left-hand side of the [blue]`?` symbol; it then compares the result obtained with the values listed in the __match-list__'s, from left to right. If the comparision finds a match with a value in a _match-list_, the associated _case-multi-expression_ is evaluted, and its result will be the final result of the selection operation.
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The match lists are optional. In that case, the position, from left to right, of the `<selector-case>` is used as match-list. Of course, that only works if the select-expression results in an integer.
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The match lists are optional. In that case, the position, from left to right, of the _selector-case_ is used as _match-list_. Of course, that only works if the _select-expression_ results in an integer.
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The `:` symbol (colon) is the separator of the selector-cases. Note that if the value of the select-expression does not match any match-list, an error will be issued. Therefore, it is strongly recommended to provide a default (multi-)expression introduced by the `::` symbol (double-colon). Also note that the default expression has no match-list.
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The [blue]`:` symbol (colon) is the separator of the selector-cases. Note that if the value of the _select-expression_ does not match any _match-list_, an error will be issued. Therefore, it is strongly recommended to provide a default (multi-)expression introduced by the [blue]`::` symbol (double-colon). Also note that the default expression has no _match-list_.
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.Examples
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[source,go]
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----
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`>>>` [blue]`1 ? {"a"} : {"b"}`
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[green]`b`
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`>>>` [blue]`10 ? {"a"} : {"b"} :: {"c"}`
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[green]`c'
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[green]`>>>` [blue]`10 ? {"a"} :[true, 2+8] {"b"} :: {"c"}`
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[green]`b`
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`>>>` [blue]`10 ? {"a"} :[true, 2+8] {"b"} ::[10] {"c"}`
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[red]`Parse Error: [1:34] case list in default clause`
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[green]`>>>` [blue]`10 ? {"a"} :[10] {x="b" but x} :: {"c"}`
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[green]`b`
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`>>>` [blue]`10 ? {"a"} :[10] {x="b"; x} :: {"c"}`
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[green]`b`
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`>>>` [blue]`10 ? {"a"} : {"b"}`
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`>>>` [blue]`1 ? {"a"} : {"b"}` +
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[green]`b` +
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`>>>` [blue]`10 ? {"a"} : {"b"} :: {"c"}` +
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[green]`c' +
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[green]`>>>` [blue]`10 ? {"a"} :[true, 2+8] {"b"} :: {"c"}` +
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[green]`b` +
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`>>>` [blue]`10 ? {"a"} :[true, 2+8] {"b"} ::[10] {"c"}` +
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[red]`Parse Error: [1:34] case list in default clause` +
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[green]`>>>` [blue]`10 ? {"a"} :[10] {x="b" but x} :: {"c"}` +
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[green]`b` +
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`>>>` [blue]`10 ? {"a"} :[10] {x="b"; x} :: {"c"}` +
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[green]`b` +
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`>>>` [blue]`10 ? {"a"} : {"b"}` +
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[red]`Eval Error: [1:3] no case catches the value (10) of the selection expression`
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----
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=== Variable default value [blue]`??` and [blue]`?=`
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The left operand of these two operators must be a variable. The right operator can be any expression. They return the value of the variable if this is define; otherwise they return the value of the right expression.
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IMPORTANT: If the left variable is defined, the right expression is not evuated at all.
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The [blue]`??` do not change the status of the left variable.
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The [blue]`?=` assigns the calculated value of the right expression to the left variable.
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.Examples
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`>>>` [blue]`var ?? (1+2)`'
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[green]`3` +
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`>>>` [blue]`var` +
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[red]`Eval Error: undefined variable or function "var"` +
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`>>>` [blue]`var ?= (1+2)` +
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[green]`3` +
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`>>>` [blue]`var` +
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[green]`3`
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NOTE: These operators have a high priority, in particular higher than the operator [blue]`=`.
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== Priorities of operators
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The table below shows all supported operators by decreasing priorities.
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