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FOBO Data Maker

Expressions & the Fn library

Forms can carry expressions in three places:

  • visibleWhen (field or layout group) — a boolean; the element shows when it’s true.
  • calculatedExpression (field) — derives the field’s value; the field becomes read-only.
  • validation expression rules — a boolean that must hold.

Expressions reference field names as variables and call functions from the Fn library. The same expressions run server-side and in the web renderer (compiled to JS — compiled.json in a .dmf v3), so results match.

age >= 18 && country == "FR"
Upper(Trim(first_name))
If(LenStr(notes) > 0, "has notes", "empty")
DiffDays(end_date, start_date) > 7

Operators

  • Comparison: == != < <= > >=
  • Logical: && || !
  • Arithmetic: + - * / %
  • Null-coalesce: ??
  • Ternary: cond ? a : b

Both Fn.Upper(x) and the bare Upper(x) forms are accepted.

The Fn library

All functions are pure and run identically on the server and in the browser (the JS port is fn.js).

Strings — Concat(a,b), Concat3, Concat4, Lower(s), Upper(s), Trim(s), TitleCase(s), Left(s,n), Right(s,n), Pad(s,width,padChar), PadRight(s,width,padChar), LenStr(s).

Regex — Match(input,pattern), Capture(input,pattern,group), Replace(input,pattern,replacement) (case-insensitive).

Convert — ToInt(s), ToLong(s), IntToStr0(n), IntToStr(n,format).

Branch — If(cond,then,else), Coalesce(a,b), Coalesce3(a,b,c), Switch(v,case,result,default), Switch2, Switch3.

Arrays — Split(input,sep), Join(arr,sep), At(arr,i), First(arr), Last(arr), Take(arr,n), Skip(arr,n), Slice(arr,start,count), IndexOf(arr,v), Where(arr,v), Except(arr,v), Insert(arr,i,v), Remove(arr,i), Filter(arr,v), Reverse(arr), Sort(arr), Distinct(arr), Map(arr,prefix,suffix), Union(a,b), Intersect(a,b), SetExcept(a,b).

Aggregates — Len(arr), Any(arr,v), All(arr,v), CountOf(arr,v), Min(a,b), Max(a,b), MinD(a,b), MaxD(a,b).

Dates (strings YYYY-MM-DD, UTC) — Year(s), Month(s), Day(s), AddDays(s,days), DiffDays(a,b), Today(), Now().

Building your own evaluator

If you render forms outside .NET/JS, you’ll need to evaluate these expressions to support visibleWhen/calculatedExpression. Two options:

  1. Use the compiled JS. A .dmf v3 ships compiled.json mapping each expression key to a ready JS function string (function(v){ return …; }, where v is the values bag). Pair it with a port of the Fn library (Dm.*) and you don’t need to parse the DSL at all.
  2. Parse the DSL yourself. Implement the operators above + the Fn functions you use. Match the case-insensitive string semantics and UTC date handling to stay consistent with the server.

If you can’t evaluate an expression, fail open for visibleWhen — show the field rather than hide it. For calculatedExpression, emit the field empty (or omit it) and surface that it couldn’t be computed; never present a calculated field as an editable input, and never substitute a stale or guessed value. A blank, clearly-uncomputed field is honest; an editable one invites a human to overwrite a value the form author meant to derive.