Add ReadJsonOperator

[illusional]

Then you could add a ReadJsonOperator with the following diff:

Index: janis_core/operators/selectors.py
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- janis_core/operators/selectors.py	(revision 96718bc123b9dc556ad1a56d476bf3da2804dc77)
+++ janis_core/operators/selectors.py	(date 1611704151813)
@@ -221,6 +221,13 @@
 
         return ReadContents(self)
 
+    def read_json(self):
+        # could always check self.returntype() and decide to use read_json or ParseJson
+
+        from janis_core.operators.standard import ReadJsonOperator
+
+        return ReadJsonOperator(self)
+
     def __getitem__(self, item):
         from janis_core.operators.operator import IndexOperator
 
Index: janis_core/operators/standard.py
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- janis_core/operators/standard.py	(revision 96718bc123b9dc556ad1a56d476bf3da2804dc77)
+++ janis_core/operators/standard.py	(date 1611704003239)
@@ -42,6 +42,37 @@
             return f.read()
 
 
+class ReadJsonOperator(Operator):
+    @staticmethod
+    def friendly_signature():
+        return f"File -> Dict[str, any]"
+
+    def evaluate(self, inputs):
+        file = self.evaluate_arg(self.args[0], inputs)
+        from json import load
+
+        with open(file) as f:
+            return load(f)
+
+    def to_wdl(self, unwrap_operator, *args):
+        f = unwrap_operator(self.args[0])
+        return f"read_json({f})"
+
+    def to_cwl(self, unwrap_operator, *args):
+        fp = unwrap_operator(self.args[0])
+        return f"JSON.parse({fp}.contents)"
+
+    def requires_contents(self):
+        return True
+
+    def argtypes(self) -> List[DataType]:
+        return [File()]
+
+    def returntype(self):
+        # dictionary?
+        return String
+
+
 class JoinOperator(Operator):
     def __init__(self, iterable, separator):
         super().__init__(iterable, separator)

And bobs your uncle, you can do something like this:

t = CommandToolBuilder(
    tool="test_readcontents",
    base_command=["echo", "1"],
    inputs=[ToolInput("inp", File)],
    outputs=[
        ToolOutput(
            "out_json", String, selector=InputSelector("inp").read_json()["out"]
        ),
    ],
    container=None,
    version="-1",
)

task test_readcontents {
  # ...
  output {
    String out_json = read_json(inp)["out"]
  }
}