Reference: expressions

This page gives reference information for Daml expressions that are not updates.

Definitions

Use assignment to bind values or functions at the top level of a Daml file or in a contract template body.

Values

For example:

pi = 3.1415926535

The fact that pi has type Decimal is inferred from the value. To explicitly annotate the type, mention it after a colon following the variable name:

pi : Decimal = 3.1415926535

Functions

You can define functions. Here’s an example: a function for computing the surface area of a tube:

tubeSurfaceArea : Decimal -> Decimal -> Decimal 
tubeSurfaceArea r h  =
  2.0 * pi * r * h

Here you see:

• the name of the function

• the function’s type signature Decimal -> Decimal -> Decimal

  This means it takes two Decimals and returns another Decimal.

• the definition = 2.0 * pi * r * h (which uses the previously defined pi)

Arithmetic operators

Operator	Works for
+	Int, Decimal, RelTime
-	Int, Decimal, RelTime
*	Int, Decimal
/ (integer division)	Int
% (integer remainder operation)	Int
^ (integer exponentiation)	Int

The result of the modulo operation has the same sign as the dividend:

• 7 / 3 and (-7) / (-3) evaluate to 2
• (-7) / 3 and 7 / (-3) evaluate to -2
• 7 % 3 and 7 % (-3) evaluate to 1
• (-7) % 3 and (-7) % (-3) evaluate to -1

To write infix expressions in prefix form, wrap the operators in parentheses. For example, (+) 1 2 is another way of writing 1 + 2.

Comparison operators

Operator	Works for
<, <=, >, >=	Bool, Text, Int, Decimal, Party, Time
==, /=	Bool, Text, Int, Decimal, Party, Time, and identifiers of contracts stemming from the same contract template

Logical operators

The logical operators in Daml are:

• not for negation, e.g., not True == False
• && for conjunction, where a && b == and a b
• || for disjunction, where a || b == or a b

for Bool variables a and b.

If-then-else

You can use conditional if-then-else expressions, for example:

if owner == scroogeMcDuck then "sell" else "buy"

Let

To bind values or functions to be in scope beneath the expression, use the block keyword let:

doubled =
  -- let binds values or functions to be in scope beneath the expression
  let
    double (x : Int) = 2 * x
    up = 5
  in double up

You can use let inside do and scenario blocks:

blah = scenario
  do
    let
      x = 1
      y = 2
      -- x and y are in scope for all subsequent expressions of the do block,
      -- so can be used in expression1 and expression2.
    expression1
    expression2

Lastly, a template may contain a single let block.

template Iou
  with
    issuer : Party
    owner  : Party
  where
    signatory issuer

    let updateOwner o = create this with owner = o
        updateAmount a = create this with owner = a

    -- Expressions bound in a template let block can be referenced
    -- from any and all of the signatory, consuming, ensure and
    -- agreement expressions and from within any choice do blocks.

    controller owner can
      Transfer : ContractId Iou
        with newOwner : Party
        do
          updateOwner newOwner