Pattern matching

Pattern matching is the act of checking if a given expression matches a pattern. For example, let's see this snippet of code:

describeNumber :: Int -> String
describeNumber 0 = "You entered exactly zero."
describeNumber x = "You entered a different number: " ++ show x

If we load that code into the Interpreter and evaluate the following expression:

describeNumber 0

It will match with the first equation, where 0 is a LiteralPattern

[EnvBuilder] Defining function: `describeNumber`
[FunctionRuntime] Applying function: `describeNumber` with args: [ '0' ]
[FunctionRuntime] Trying to match value `0` with `0`
[FunctionRuntime] Match successful for `describeNumber` equation 0

You entered exactly zero.

But if we try with another string instead

describeNumber 10

The Interpreter will not match to 0, instead it will bind x -> 10 because x is a VariablePattern

[EnvBuilder] Defining function: `describeNumber`
[FunctionRuntime] Applying function: `describeNumber` with args: [ '10' ]
[FunctionRuntime] Trying to match value `10` with `0`
[FunctionRuntime] Could not match value `10` with `0`
[FunctionRuntime] Trying to match value `10` with `x`
[FunctionRuntime] Match successful for `describeNumber` equation 1

You entered a different number: 10

When the FunctionRuntime applies an expression to that function, the PatternMatcher tries each equation top-to-bottom until it finds the first that matches. In the second example, the catch-all variable x successfully matches the input and binds the value 10 so it can be used in the resulting expression.

Evaluation Flow

The following sequence diagram illustrates the internal process runned by the Interpreter when it evaluates an expression that requires fallback pattern matching, such as describeNumber 10.

See also

• Wikipedia - Pattern matching

• The Haskell 98 Report - Pattern matching