Thanks Eric. I've made some modifications to your code and got it working - see final section of the attached notebook. I would, however, still like to understand why using the variable 'expression' in my original notebook failed whereas pasting the evaluated form of 'expression' gave the desired result. I suspect it'll be to do with the evaluation process. But since using variables for keys in Slot is a useful thing to be able to do so I'd like to understand how it works (or, in this case, doesn't). Thanks again, Ian

I would go about it like this (there are probably refinements, but this is a working starting point):

• Create a function to map keys to types:

  MapThread[(keyType[#1] = #2) &, {keys, types}]
  

• An alternate to your toType function (I simplified it a bit, but if any of this is wrong you can figure out how to adjust it):

  typedVal[type_String, ""] := Missing["Empty string", type];
  typedVal[type_String?(StringEndsQ[{"DP", "SF"}]), val_String] := ToExpression[val];
  typedVal[type_String?(StringEndsQ[{"SCI", "U"}]), val_String] := Interpreter["Number"][val];
  typedVal[type_String, val_] := val
  

  You don't actually need to name the first argument, since we don't actually need to reference that argument's name (except for the first form, which is probably overkill anyway).

• Create a function to map the original pairs to pairs with typed values:

  toTypedPair[key_String, val_] := key -> typedVal[keyType[key], val]
  

• Apply it to ds:

  ds[All, Association@*KeyValueMap[toTypedPair]]