BQN String interpolation

example-html.bqn

string ← •Import "string.bqn"

# Poor mans templating engine

Fmt←{ 𝕨⊸string.Fmt¨<˘𝕩}

foo←∾∾⟨
  "<div>"
    "<span>{value}</span>" Fmt ↕10
    "<span class='{0}'>{1}</span>" Fmt [⟨"foo",1⟩,⟨"bar",2⟩]
  "</div>"
⟩

•Show foo 

example.bqn

"Hello {foo} {bar}!" Fmt {foo⇐"formatted" ⋄ bar⇐"world"}  # using namespaces
"Hello {} {}!"       Fmt "formatted"‿"world" # using lists
"Hello {}!"          Fmt "formatted"‿"world" # extra list values are dropped off 
"Hello {foo} {bar}!" Fmt "formatted"‿"world" # lists can still use named templates and will replace in order

string.bqn

Fmt ⇐ { 
   ch 𝕊 d: 6≡•Type 𝕩 ? # named replacement values
      s‿e←ch⊸=¨"{}" ⋄ f←(+`s+»e)⊸⊔ch # fragments
      m←f∊t←{'{'∾𝕩∾'}'}¨k←•ns.Keys d # k: keys, t: tags
      v←(m/t⊐f)⊸⊏ (d •ns.Get¨k) # v: replacement values
      ∾v⌾(m⊸/) f

  ; ch 𝕊 𝕩: # unnamed replacement values
      s‿e←ch⊸=¨"{}" ⋄ f←(+`s+»e)⊸⊔ch # fragments
      m←∾('}'=¯1↑¨f)∧'{'=1↑¨f
      t←m/f # tags
      v←(≠t)𝕩∾⟨⟩¨↕0⌈(≠t)-(≠𝕩) # replacement values
      ∾v⌾(m⊸/)f
}