Add skeletons for function implementations

byg/src/Evaluation/Function.hs

@@ -5,4 +5,12 @@ module Evaluation.Function
 import Types (Function(..), Value(..))
 
 evalFunction :: Function -> Value -> Value
-evalFunction = undefined
+evalFunction f x = case (f, x) of
+  (IsImageFilename, String _) ->
+    Bool undefined
+
+  (ConvertedImageFilename, String _) ->
+    String undefined
+
+  _ ->
+    error "unexpected combination of function and argument type"

byg/src/Evaluation/FunctionIO.hs

@@ -5,4 +5,21 @@ module Evaluation.FunctionIO
 import Types (FunctionIO(..), Value(..))
 
 evalFunctionIO :: FunctionIO -> Value -> IO Value
-evalFunctionIO = undefined
+evalFunctionIO f x = case (f, x) of
+  (ListDirectory, String _) ->
+    pure $ List undefined
+
+  (ReadTemplate, String _) ->
+    pure $ Template undefined
+
+  (ConvertImage, Tuple (Tuple (String _, String _), ImageConversionSettings _)) ->
+    pure $ Empty
+
+  (SaveFile, Tuple (String _, String _)) ->
+    pure $ Empty
+
+  (RunPandoc, String _) ->
+    pure $ String undefined
+
+  _ ->
+    error "unexpected combination of function and argument type"