How to use StringTemplate for code?

Question

Say, I want to write a MMA script using StringTemplate, so I can insert additional pieces of code / expressions wherever TemplateSlots appear. It is a bit unclear to me, however, how I should handle these slots. For example the template

tmp = StringTemplate[
  ToString[
    Unevaluated[
      Pi/2 + "`a`" + 2
    ],
    InputForm
  ]
]

gives

tmp[<|"a" -> 1|>]
(* Pi/2 + "1" + 2 *)

That is, it looks like the parameter 1 is substituted as a String whereas it should be a number. This is not the case, however. What actually happens is, that 1 gets inserted, but the quotation marks in the template remain. So how should I declare TemplateSlots in my StringTemplate? It should work with any MMA code inside the Unevaluated.

Answer 1

As an alternative to Carl's method you can use:

StringTemplate["Pi/2 + `` + 2"]["`a`"]

"Pi/2 + `a` + 2" 

so

templ = StringTemplate[
   StringTemplate["Pi/2 + `` + 2"]@"`a`"
];

templ @ <|"a" -> 2|>

"Pi/2 + 2 + 2"

but for "a" -> -1 you will get

"Pi/2 + -1 + 2"

so I'd suggest full TemplateObject mode:

TemplateObject[
    TemplateExpression[
        ToString[Unevaluated[Pi/2 + TemplateSlot["a"] + 2], InputForm]
    ]
] @ <|"a" -> -1|>

"Pi/2 - 1 + 2"

or

TemplateObject[
    Hold[Pi/2 + TemplateSlot["a"] + 2]
  , CombinerFunction -> (ToString[#, InputForm] &@*Apply[Unevaluated])
] @ <|"a" -> -1|>