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I'm trying to define a local function within another function but can't seem to get the syntax right. I have the following:

foo : Int -> Bool
foo i =
  let bar j = j < 5
  bar i

But I'm getting a syntax error on the bar. What is the correct syntax to define such a function?

GeorgS
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1 Answers1

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For ordinary local function bindings, and variable bindings with let in general, you have to use in after all functions/variables have been defined:

  let bar j = j < 5
  in bar i

A let without in can be used within a do block only, in which case the variable is defined for the rest of the do block:

blah = do
  let bar j = j < 5
  assert $ not $ bar 5
  pure $ bar 5

However, that can only be used where it is valid to use do. Otherwise, use in.

A where clause can also be attached to a definition, in lieu of let:

foo i = bar i
  where bar j = j < 5

This use of where is unrelated to its role in template syntax.

Note that where, while it is nicer for some cases, is pretty restrictive about where (sorry) it can be used, whereas (sorry again) let/in works anywhere an expression is allowed:

foo i =
  let bar j = j < 5
  in bar (let q = 2 in i + q)

See daml docs on let and this discussion of when to use let or where.

Stephen Compall
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