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Smart constructors

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Smart constructors

This is an introduction to a programming idiom for placing extra constraints on the construction of values by using smart constructors.

Sometimes you need guarantees about the values in your program beyond what can be accomplished with the usual type system checks. Smart constructors can be used for this purpose.

Consider the following problem: we want to be able to specify a data type for electronic resistors. The resistors come in two forms, metal and ceramic. Resistors are labelled with a number of bands, from 4 to 8.

We'd like to be able to

  • ensure only resistors with the right number of bands are constructed.


1 Runtime checking : smart constructors

1.1 A first attempt

Code up the a typical data type describing a resistor value:

data Resistor = Metal   Bands
              | Ceramic Bands 
   deriving Show
type Bands = Int

This has a problem however, that the constructors of type Resistor are unable to check that only bands of size 4 to 8 are built. It is quite legal to say:

*Main> :t Metal 23
Metal 23 :: Resistor

for example.

1.2 Smart(er) constructors

Smart constructors are just functions that build values of the required type, but perform some extra checks when the value is constructed, like so:

metalResistor :: Bands -> Resistor
metalResistor n | n < 4 || n > 8 = error "Invalid number of resistor bands" 
                | otherwise      = Metal n

This function behaves like the constructor Metal, but also performs a check. This check will be carried out at runtime, once, when the value is built.

Running this code:

*Main> metalResistor 4
Metal 4
*Main> metalResistor 7
Metal 7

*Main> metalResistor 9
*** Exception: Invalid number of resistor bands
*Main> metalResistor 0
*** Exception: Invalid number of resistor bands

One extra step has to be made though, to make the interface safe. When exporting the type Resistor we need to hide the (unsafe) constructors, and only export the smart constructors, otherwise a reckless user could bypass the smart constructor:

module Resistor (
       Resistor,       -- abstract, hiding constructors
       metalResistor,  -- only way to build a metal resistor
  ) where

1.3 Using assertions

Hand-coding error messages can be tedious when used often. Instead we can use the assert function, provided (at least with GHC). We rewrite the smart constructor as:

metalResistor :: Bands -> Resistor
metalResistor n = assert (n >= 4 && n <= 8) $ Metal n

And now obtain more detailed error messages, automatically generated for us:

*Main> metalResistor 0
*** Exception: A.hs:4:18-23: Assertion failed

We at least now are given the line and column in which the error occured.

2 Compile-time checking : the type system

2.1 Enforcing the constraint statically

There are other ways to obtain numerical checks like this. The most interesting are probably the static checks that can be done with Type arithmetic, that enforce the number of bands at compile time, rather than runtime, by lifting the band count into the type level.

Todo: example

2.2 Extensions

Further checks can be obtained by separating the metal and ceramic values on the type level, so no function that takes a metal resistor can be accidentally passed a ceramic one.

A newtype is useful for this:

newtype MetalResistor   = Metal   Bands
newtype CeramicResistor = Ceramic Bands

now, a function of resistors must have either a MetalResistor type, or a CeramicResistor type:

foo :: MetalResistor -> Int
foo (MetalResistor n) = n

You can't write a function over both resistor types (other than a purely polymorphic function).

2.3 Related work

These ideas are also discussed on the old wiki here and also here (for compile-time unit analysis error catching at the type level). More here too.

In general, the more information you place on the type level, the more static checks you get -- and thus less chance for bugs.