if then else expressions

SPEC.md

@@ -413,6 +413,7 @@ $expression = $expression '(' ($expression (',' $expression)*)? ')'
 $expression = '!' $expression
 $expression = '+' $expression
 $expression = '-' $expression
+$expression = if $expression then $expression else $expression
 $expression = $expression '*' $expression
 $expression = $expression '%' $expression
 $expression = $expression '/' $expression
@@ -510,6 +511,25 @@ Below are the valid results for operators on types.  Any combination not in the
 ||`+`|`Int`|`Int`||
 ||`!`|`Boolean`|`Boolean`||
 
+#### If then else
+
+This is an operator that takes three arguments, a condition expression, an if-true expression and an if-false expression. The condition is always evaluated. If the condition is true then the if-true value is evaluated and returned. If the condition is false, the if-false expression is evaluated and returned. The return type of the if-then-else should be the same, regardless of which side is evaluated or runtime problems might occur.
+
+Examples:
+ - Choose whether to say "good morning" or "good afternoon":
+```
+Boolean morning = ...
+String greeting = "good " + if morning then "morning" else "afternoon"
+```
+- Choose how much memory to use for a task:
+```
+Int array_length = length(array)
+runtime {
+  memory: if array_length > 100 then "16GB" else "8GB"
+} 
+```
+
+
 ### Operator Precedence Table
 
 :pig2: [Cromwell supported](https://github.com/broadinstitute/cromwell#wdl-support) :white_check_mark:

grammar.hgr

@@ -25,6 +25,8 @@ grammar {
     r'output(?!{%_identifier_follow%})' -> output(:output)
     r'as(?!{%_identifier_follow%})' -> :as
     r'if(?!{%_identifier_follow%})' -> :if
+    r'then(?!{%_identifier_follow%})' -> :then
+    r'else(?!{%_identifier_follow%})' -> :else
     r'while(?!{%_identifier_follow%})' -> :while
     r'runtime(?!{%_identifier_follow%})' -> :runtime
     r'scatter(?!{%_identifier_follow%})' -> :scatter @scatter
@@ -101,19 +103,23 @@ grammar {
       r'\}' -> :rbrace %pop
       r'\[' -> :lsquare
       r'\]' -> :rsquare
-      r'=' -> :equal
       r'\+' -> :plus
       r'\*' -> :asterisk
       r'[0-9]+' -> :integer
       r'(true|false)(?!{%_identifier_follow%})' -> :boolean
+      r'if' -> :if
+      r'else' -> :else
+      r'then' -> :then
       r'{%_type%}(?!{%_identifier_follow%})' -> :type
       r'{%_identifier%}' -> :identifier
 
+
       # Expression tokens
       r':' -> :colon
       r',' -> :comma
       r'\.' -> :dot
       r'==' -> :double_equal
+      r'=' -> :equal
       r'\|\|' -> :double_pipe
       r'\&\&' -> :double_ampersand
       r'!=' -> :not_equal
@@ -163,12 +169,8 @@ grammar {
     }
     mode<scatter> {
       r'\s+' -> null
-      r'\)' -> :rparen %pop
       r'\(' -> :lparen
-      r'\.' -> :dot
-      r'\[' -> :lsquare
-      r'\]' -> :rsquare
-      r'in(?!{%_identifier_follow%})' -> :in
+      r'in(?!{%_identifier_follow%})' -> :in %pop
       r'{%_identifier%}' -> :identifier
     }
     mode<raw_command> {
@@ -202,6 +204,9 @@ grammar {
       r'\+' -> :plus
       r'\*' -> :asterisk
       r'[0-9]+' -> :integer
+      r'if' -> :if
+      r'else' -> :else
+      r'then' -> :then
       r'{%_identifier%}(?=\s*=)' -> :cmd_attr_hint[] :identifier
       r'(true|false)(?!{%_identifier_follow%})' -> :boolean
       r'{%_type%}(?!{%_identifier_follow%})' -> :type
@@ -428,6 +433,7 @@ grammar {
       (*:left) $e = :lsquare list($e, :comma) :rsquare -> ArrayLiteral(values=$1)
       (*:left) $e = :lbrace list($map_kv, :comma) :rbrace -> MapLiteral(map=$1)
       (*:left) $e = :lparen list($e, :comma) :rparen -> TupleLiteral(values=$1)
+      (*:left) $e = :if $e :then $e :else $e -> TernaryIf(cond=$1, iftrue=$3, iffalse=$5)
       $e = :string
       $e = :identifier
       $e = :boolean